In addition, two parts of the country that are disproportionately Black or Latino — Washington, D.C., and Puerto Rico — have no Senate representation. Washington has more residents than Vermont or Wyoming, and Puerto Rico has more residents than 20 states. As a result, the Senate gives a political voice to white Americans that is greater than their numbers.

The House of Representatives has a more equitable system for allocating political power. It divides the country into 435 districts, each with a broadly similar number of people (currently about 760,000). Still, House districts have two features that can cause the chamber’s makeup not to reflect national opinion, and both of them have become more significant in recent years.

The first is well known: gerrymandering. State legislatures often draw district boundaries and in recent years have become more aggressive about drawing them in partisan ways. In Illinois, for example, the Democrats who control the state government have packed Republican voters into a small number of House districts, allowing most other districts to lean Democratic. In Wisconsin, Republicans have done the opposite.

Because Republicans have been more forceful about gerrymandering than Democrats, the current House map slightly favors Republicans, likely by a few seats. At the state level, Republicans have been even bolder. Gerrymandering has helped them dominate the state legislatures in Michigan, North Carolina and Ohio, even though the states are closely divided.

Still, gerrymandering is not the only reason that House membership has become less reflective of national opinion in recent years. It may not even be the biggest reason, according to Jonathan A. Rodden, a political scientist at Stanford University. Geographic sorting is.

“Without a doubt, gerrymandering makes things worse for the Democrats,” Mr. Rodden has written, “but their underlying problem can be summed up with the old real estate maxim: location, location, location.” The increasing concentration of Democratic voters into large metro areas means that even a neutral system would have a hard time distributing these tightly packed Democratic voters across districts in a way that would allow the party to win more elections.

Instead, Democrats now win many House elections in urban areas by landslides, wasting many votes. In 2020, only 21 Republican House candidates won their elections by at least 50 percentage points. Forty-seven Democrats did.

Looking at where many of these elections occurred helps make Mr. Rodden’s point. The landslide winners included Representative Diana DeGette in Denver; Representative Jerry Nadler in New York City; Representative Jesús García in Chicago; Representative Donald Payne Jr. in northern New Jersey; and Representative Barbara Lee in Oakland, Calif. None of those districts are in states where Republicans have controlled the legislative boundaries, which means that they were not the result of Republican gerrymandering.

Again and again, geographic sorting has helped cause a growing disconnect between public opinion and election results, and this disconnect has shaped the Supreme Court, as well. The court’s membership at any given time is dictated by the outcomes of presidential and Senate elections over the previous few decades. And if elections reflected popular opinion, Democratic appointees would dominate the court.

Every current justice has been appointed during one of the past nine presidential terms, and a Democrat has won the popular vote in seven of those nine and the presidency in five of the nine. Yet the court is now dominated by a conservative, six-member majority.

There are multiple reasons (including Ruth Bader Ginsburg’s decision not to retire in 2014 when a Democratic president and Senate could have replaced her). But the increasingly undemocratic natures of both the Electoral College and Senate play crucial roles.

Mr. Trump was able to appoint three justices despite losing the popular vote. (Mr. Bush is a more complex case, having made his court appointments after he won re-election and the popular vote in 2004.) Similarly, if Senate seats were based on population, none of Mr. Trump’s nominees — Justices Neil M. Gorsuch, Brett M. Kavanaugh and Amy Coney Barrett — would likely have been confirmed, said Michael J. Klarman, a law professor at Harvard. Senate Republicans also would not have been able to block Mr. Obama from filling a court seat during his final year in office.

Even Justice Clarence Thomas’s 1991 confirmation relied on the Senate’s structure: The 52 senators who voted to confirm him represented a minority of Americans.

The current court’s approach has magnified the disconnect between public opinion and government policy, because Republican-appointed justices have overruled Congress on some major issues. The list includes bills on voting rights and campaign finance that earlier Congresses passed along bipartisan lines. This term, the court issued rulings on abortion, climate policy and gun laws that seemed to be inconsistent with majority opinion, based on polls.

“The Republican justices wouldn’t say this and may not believe it,” Mr. Klarman said, “but everything they’ve done translates into a direct advantage for the Republican Party.”

In response to the voting rights decision in 2013, Republican legislators in several states have passed laws making it more difficult to vote, especially in heavily Democratic areas. They have done so citing the need to protect election security, even though there has been no widespread fraud in recent years.

For now, the electoral effect of these decisions remains uncertain. Some analysts point out that the restrictions have not yet been onerous enough to hold down turnout. In the 2020 presidential election, the percentage of eligible Americans who voted reached the highest level in at least a century.

Other experts remain concerned that the new laws could ultimately swing a close election in a swing state. “When you have one side gearing up to say, ‘How do we stop the enemy from voting?’ that is dangerous to a democracy,” Ms. Anderson, the Emory professor, said.

An upcoming Supreme Court case may also allow state legislatures to impose even more voting restrictions. The court has agreed to hear a case in which Republican legislators in North Carolina argue that the Constitution gives them, and not state courts, the authority to oversee federal elections.

In recent years, state courts played an important role in constraining both Republican and Democratic legislators who tried to draw gerrymandered districts that strongly benefited one party. If the Supreme Court sides with the North Carolina Legislature, gerrymandering might increase, as might laws establishing new barriers to voting.

Amplifying the Election Lies

If the only challenges to democracy involved these chronic, long-developing forces, many experts would be less concerned than they are. American democracy has always been flawed, after all.

But the slow-building ways in which majority rule is being undermined are happening at the same time that the country faces an immediate threat that has little precedent. A growing number of Republican officials are questioning a basic premise of democracy: That the losers of an election are willing to accept defeat.

The roots of the modern election-denier movement stretch back to 2008. When Mr. Obama was running for president and after he won, some of his critics falsely claimed that his victory was illegitimate because he was born in Kenya rather than Hawaii. This movement became known as birtherism, and Mr. Trump was among its proponents. By making the claims on Fox News and elsewhere, he helped transform himself from a reality television star into a political figure.

When he ran for president himself in 2016, Mr. Trump made false claims about election fraud central to his campaign. In the Republican primaries, he accused his closest competitor for the nomination, Senator Ted Cruz, of cheating. In the general election against Hillary Clinton, Mr. Trump said he would accept the outcome only if he won. In 2020, after Mr. Biden won, the election lies became Mr. Trump’s dominant political message.

His embrace of these lies was starkly different from the approach of past leaders from both parties. In the 1960s, Reagan and Barry Goldwater ultimately isolated the conspiracists of the John Birch Society. In 2000, Al Gore urged his supporters to accept George W. Bush’s razor-thin victory, much as Richard Nixon had encouraged his supporters to do so after he narrowly lost to John F. Kennedy in 1960. In 2008, when a Republican voter at a rally described Mr. Obama as an Arab, Senator John McCain, the Republican nominee and Mr. Obama’s opponent, corrected her.

Mr. Trump’s promotion of the falsehoods, by contrast, turned them into a central part of the Republican Party’s message. About two-thirds of Republican voters say that Mr. Biden did not win the 2020 election legitimately, according to polls. Among Republican candidates running for statewide office this year, 47 percent have refused to accept the 2020 result, according to a FiveThirtyEight analysis.

Most Republican politicians who have confronted Mr. Trump, on the other hand, have since lost their jobs or soon will. Of the 10 House Republicans who voted to impeach him for his role in the Jan. 6 attack, for example, eight have since decided to retire or lost Republican primaries, including Representative Liz Cheney of Wyoming.

“By any indication, the Republican Party — upper level, midlevel and grass roots — is a party that can only be described as not committed to democracy,” Mr. Levitsky said. He added that he was significantly more concerned about American democracy than when his and Mr. Ziblatt’s book, “How Democracies Die,” came out in 2018.

Juan José Linz, a political scientist who died in 2013, coined the term “semi-loyal actors” to describe political officials who typically do not initiate attacks on democratic rules or institutions but who also do not attempt to stop these attacks. Through their complicity, these semi-loyal actors can cause a party, and a country, to slide toward authoritarianism.

That’s what happened in Europe in the 1930s and in Latin America in the 1960s and ’70s. More recently, it has happened in Hungary. Now there are similar signs in the United States.

Often, even Republicans who cast themselves as different from Mr. Trump include winking references to his conspiracy theories in their campaigns, saying that they, too, believe “election integrity” is a major problem. Gov. Glenn Youngkin of Virginia and Gov. Ron DeSantis of Florida, for example, have both recently campaigned on behalf of election deniers.

In Congress, Republican leaders have largely stopped criticizing the violent attack on the Capitol. Representative Kevin McCarthy, the Republican House leader, has gone so far as to signal his support for colleagues — like Representative Marjorie Taylor Greene of Georgia — who have used violent imagery in public comments. Ms. Greene, before being elected to Congress, said that she supported the idea of executing prominent Democrats.

“When mainstream parties tolerate these guys, make excuses for them, protect them, that’s when democracy gets in trouble,” Mr. Levitsky said. “There have always been Marjorie Taylor Greenes. What I pay closer attention to is the behavior of the Kevin McCarthys.”

The party’s growing acceptance of election lies raises the question of what would happen if Mr. Trump or another future presidential nominee tried to replay his 2020 attempt to overturn the result.

In 11 states this year, the Republican nominee for secretary of state, a position that typically oversees election administration, qualifies as an “election denier,” according to States United Action, a research group. In 15 states, the nominee for governor is a denier, and in 10 states, the attorney general nominee is.

The growth of the election-denier movement has created a possibility that would have seemed unthinkable not so long ago. It remains unclear whether the loser of the next presidential election will concede or will instead try to overturn the outcome.

‘There Is a Crisis Coming’

There are still many scenarios in which the United States will avoid a democratic crisis.

In 2024, Mr. Biden could win re-election by a wide margin — or a Republican other than Mr. Trump could win by a wide margin. Mr. Trump might then fade from the political scene, and his successors might choose not to embrace election falsehoods. The era of Republican election denial could prove to be brief.

It is also possible that Mr. Trump or another Republican nominee will try to reverse a close defeat in 2024 but will fail, as happened in 2020. Then, Brad Raffensperger, Georgia’s Republican secretary of state, rebuffed Mr. Trump after he directed him to “find 11,780 votes,” and the Supreme Court refused to intervene, as well. More broadly, Mitch McConnell, the Republican leader in the Senate, recently said that the United States had “very little voter fraud.”

If a Republican were again to try to overturn the election and to fail, the movement might also begin to fade.

But many democracy experts worry that these scenarios may be wishful thinking. Mr. Trump’s most likely successors as party leader also make or tolerate false claims about election fraud. The movement is bigger than one person — and arguably always has been: Some of the efforts to make voting more onerous, which are generally justified with false suggestions of widespread voter fraud, predated Mr. Trump’s 2016 candidacy.

To believe that Republicans will not overturn a close presidential loss in coming years seems to depend on ignoring the public positions of many Republican politicians. “The scenarios by which we don’t have a major democracy crisis by the end of the decade seem rather narrow,” Mr. Mounk of Johns Hopkins said.

And Mr. Levitsky said, “It’s not clear how the crisis is going to manifest itself, but there is a crisis coming.” He added, “We should be very worried.”

The most promising strategy for avoiding an overturned election, many scholars say, involves a broad ideological coalition that isolates election deniers. But it remains unclear how many Republican politicians would be willing to join such a coalition.

It is also unclear whether Democratic politicians and voters are interested in making the compromises that would help them attract more voters. Many Democrats have instead embraced a purer version of liberalism in recent years, especially on social issues. This shift to the left has not prevented the party from winning the popular vote in presidential elections. But it has hurt Democrats outside of major metropolitan areas and, by extension, in the Electoral College and congressional elections.

If Democrats did control both the White House and Congress — and by more than a single vote, as they now do in the Senate — they have signaled that they would attempt to pass legislation to address both the chronic and acute threats to democracy.

The House last year passed a bill to protect voting rights and restrict gerrymandering. It died in the Senate partly because it included measures that even some moderate Democrats believed went too far, such as restrictions on voter identification laws, which many other democracies around the world have.

The House also passed a bill to grant statehood to Washington, D.C., which would reduce the Senate’s current bias against metropolitan areas and Black Americans. The United States is currently in its longest stretch without having admitted a new state.

Democracy experts have also pointed to other possible solutions to the growing disconnect between public opinion and government policy. Among them is an expansion of the number of members in the House of Representatives, which the Constitution allows Congress to do — and which it regularly did until the early 20th century. A larger House would create smaller districts, which in turn could reduce the share of uncompetitive districts.

Other scholars favor proposals to limit the Supreme Court’s authority, which the Constitution also allows and which previous presidents and Congresses have done.

In the short term, these proposals would generally help the Democratic Party, because the current threats to majority rule have mostly benefited the Republican Party. In the long term, however, the partisan effects of such changes are less clear.

The history of new states makes this point: In the 1950s, Republicans initially supported making Hawaii a state, because it seemed to lean Republican, while Democrats said that Alaska had to be included, too, also for partisan reasons. Today, Hawaii is a strongly Democratic state, and Alaska is a strongly Republican one. Either way, the fact that both are states has made the country more democratic.

Over the sweep of history, the American government has tended to become more democratic, through women’s suffrage, civil rights laws, the direct election of senators and more. The exceptions, like the post-Reconstruction period, when Black Southerners lost rights, have been rare. The current period is so striking partly because it is one of those exceptions.

“The point is not that American democracy is worse than it was in the past,” Mr. Mounk said. “Throughout American history, the exclusion of minority groups, and African Americans in particular, was much worse than it is now.”

“But the nature of the threat is very different than in the past,” he said.

The makeup of the federal government reflects public opinion less closely than it once did. And the chance of a true constitutional crisis — in which the rightful winner of an election cannot take office — has risen substantially. That combination shows that American democracy has never faced a threat quite like the current one.

Nick Corasaniti, Max Fisher, Adam Liptak, Jennifer Medina, Jeremy W. Peters and Ian Prasad Philbrick contributed reporting.

David Leonhardt writes The Morning, The Times’s flagship daily newsletter. He has previously been an Op-Ed columnist, Washington bureau chief, co-host of “The Argument” podcast, founding editor of The Upshot section and a staff writer for The Times Magazine. In 2011, he received the Pulitzer Prize for commentary.