Donald J. Trump's election has raised a question that few Americans ever imagined asking: Is our democracy in danger?

With the possible exception of the Civil War, American democracy has never collapsed; indeed, no democracy as rich or as established as America's ever has. Yet past stability is no guarantee of democracy's future survival.

We have spent two decades studying the emergence and breakdown of democracy in Europe and Latin America. Our research points to several warning signs.

The clearest warning sign is the ascent of anti-democratic politicians into mainstream politics.

Drawing on a close study of democracy's demise in 1930s Europe, the eminent political scientist Juan J. Linz designed a "litmus test" to identify anti-democratic politicians. His indicators include a failure to reject violence unambiguously, a readiness to curtail rivals' civil liberties, and the denial of the legitimacy of elected governments.

Mr. Trump tests positive.

In the campaign, he encouraged violence among supporters; pledged to prosecute Hillary Clinton; threatened legal action against unfriendly media; and suggested that he might not accept the election results.This anti-democratic behavior has continued since the election.

With the false claim that he lost the popular vote because of "millions of people who voted illegally," Mr. Trump openly challenged the legitimacy of the electoral process.

At the same time, he has been remarkably dismissive of United States intelligence agencies' reports of Russian hacking to tilt the election in his favor.

Mr. Trump is not the first American politician with authoritarian tendencies. (Other notable authoritarians include Gov. Huey Long of Louisiana and Senator Joseph McCarthy of Wisconsin.)

But he is the first in modern American history to be elected president.

This is not necessarily because Americans have grown more authoritarian (the United States electorate has always had an authoritarian streak). Rather it's because the institutional filters that we assumed would protect us from extremists, like the party nomination system and the news media, failed.

Many Americans are not overly concerned about Mr. Trump's authoritarian inclinations because they trust our system of constitutional checks and balances to constrain him.Yet the institutional safeguards protecting our democracy may be less effective than we think.

A well-designed constitution is not enough to ensure a stable democracy — a lesson many Latin American independence leaders learned when they borrowed the American constitutional model in the early 19th century, only to see their countries plunge into chaos.

Democratic institutions must be reinforced by strong informal norms.

Like a pickup basketball game without a referee, democracies work best when unwritten rules of the game, known and respected by all players, ensure a minimum of civility and cooperation. Norms serve as the soft guardrails of democracy, preventing political competition from spiraling into a chaotic, no-holds-barred conflict.Among the unwritten rules that have sustained American democracy are partisan self-restraint and fair play.

For much of our history, leaders of both parties resisted the temptation to use their temporary control of institutions to maximum partisan advantage, effectively under-utilizing the power conferred by those institutions.

There existed a shared understanding, for example, that anti-majoritarian practices like the Senate filibuster would be used sparingly, that the Senate would defer (within reason) to the president in nominating Supreme Court justices, and that votes of extraordinary importance — like impeachment — required a bipartisan consensus.

Such practices helped to avoid a descent into the kind of partisan fight to the death that destroyed many European democracies in the 1930s.

Yet norms of partisan restraint have eroded in recent decades.

House Republicans' impeachment of Bill Clinton in 1998 abandoned the idea of bipartisan consensus on impeachment. The filibuster, once a rarity, has become a routine tool of legislative obstruction. As the political scientists Thomas Mann and Norman Ornstein have shown, the decline of partisan restraint has rendered our democratic institutions increasingly dysfunctional.

Republicans' 2011 refusal to raise the debt ceiling, which put America's credit rating at risk for partisan gain, and the Senate's refusal this year to consider President Obama's Supreme Court nominee — in essence, allowing the Republicans to steal a Supreme Court seat — offer an alarming glimpse at political life in the absence of partisan restraint.Norms of presidential restraint are also at risk.

The Constitution's ambiguity regarding the limits of executive authority can tempt presidents to try and push those limits. Although executive power has expanded in recent decades, it has ultimately been reined in by the prudence and self-restraint of our presidents.

Unlike his predecessors, Mr. Trump is a serial norm-breaker.

There are signs that Mr. Trump seeks to diminish the news media's traditional role by using Twitter, video messages and public rallies to circumvent the White House press corps and communicate directly with voters — taking a page out of the playbook of populist leaders like Silvio Berlusconi in Italy, Hugo Chávez in Venezuela and Recep Tayyip Erdogan in Turkey.

An even more basic norm under threat today is the idea of legitimate opposition.

In a democracy, partisan rivals must fully accept one another's right to exist, to compete and to govern. Democrats and Republicans may disagree intensely, but they must view one another as loyal Americans and accept that the other side will occasionally win elections and lead the country. Without such mutual acceptance, democracy is imperiled.

Governments throughout history have used the claim that their opponents are disloyal or criminal or a threat to the nation's way of life to justify acts of authoritarianism.The idea of legitimate opposition has been entrenched in the United States since the early 19th century, disrupted only by the Civil War. That may now be changing, however, as right-wing extremists increasingly question the legitimacy of their liberal rivals.

During the last decade, Ann Coulter wrote best-selling books describing liberals as traitors, and the "birther" movement questioned President Obama's status as an American.

Such extremism, once confined to the political fringes, has now moved into the mainstream.

In 2008, the Republican vice-presidential candidate Sarah Palin linked Barack Obama to terrorism. This year, the Republican Party nominated a birther as its presidential candidate. Mr. Trump's campaign centered on the claim that Hillary Clinton was a criminal who should be in jail; and "Lock her up!" was chanted at the Republican National Convention. In other words, leading Republicans — including the president-elect — endorsed the view that the Democratic candidate was not a legitimate rival.

The risk we face, then, is not merely a president with illiberal proclivities — it is the election of such a president when the guardrails protecting American democracy are no longer as secure.American democracy is not in imminent danger of collapse.

If ordinary circumstances prevail, our institutions will most likely muddle through a Trump presidency.

It is less clear, however, how democracy would fare in a crisis. In the event of a war, a major terrorist attack or large-scale riots or protests — all of which are entirely possible — a president with authoritarian tendencies and institutions that have come unmoored could pose a serious threat to American democracy. We must be vigilant. The warning signs are real.

This article appeared in the New York Times on December 16, 2016 . The writers are professors of government at Harvard University.

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December 17, 2016