Two political scientists specializing in how democracies decay and die have compiled four warning signs to determine if a political leader is a dangerous authoritarian:

1. The leader shows only a weak commitment to democratic rules. 2. He or she denies the legitimacy of opponents. 3. He or she tolerates violence. 4. He or she shows some willingness to curb civil liberties or the media.

“A politician who meets even one of these criteria is cause for concern,” Steven Levitsky and Daniel Ziblatt, both professors at Harvard, write in their important new book, “How Democracies Die,” which will be released next week.

“With the exception of Richard Nixon, no major-party presidential candidate met even one of these four criteria over the last century,” they say, which sounds reassuring. Unfortunately, they have one update: “Donald Trump met them all.”

A survey that year found that the Venezuelan public overwhelmingly believed that “democracy is always the best form of government,” with only one-quarter saying that authoritarianism is sometimes preferable. Yet against their will, Venezuelans slid into autocracy.

“This is how democracies now die,” Levitsky and Ziblatt write. “Democratic backsliding today begins at the ballot box.”

We tend to assume that the threat to democracies comes from coups or violent revolutions, but the authors say that in modern times, democracies are more likely to wither at the hands of insiders who gain power initially through elections. That’s what happened, to one degree or another, in Russia, the Philippines, Turkey, Venezuela, Ecuador, Hungary, Nicaragua, Sri Lanka, Ukraine, Poland and Peru.

Venezuela was a relatively prosperous democracy, for example, when the populist demagogue Hugo Chávez tapped the frustrations of ordinary citizens to be elected president in 1998.

Likewise, the authors say, no more than 2 percent of Germans or Italians joined the Nazi or Fascist Parties before they gained power, and early on there doesn’t seem to have been clear majority support for authoritarianism in either Germany or Italy. But both Hitler and Mussolini were shrewd demagogues who benefited from the blindness of political insiders who accommodated them.

Let me say right here that I don’t for a moment think the United States will follow the path of Venezuela, Germany or Italy. Yes, I do see in Trump these authoritarian tendencies — plus a troubling fondness for other authoritarians, like Vladimir Putin in Russia and Rodrigo Duterte in the Philippines — but I’m confident our institutions are stronger than Trump.

It’s true that he has tried to undermine institutions and referees of our political system: judges, the Justice Department, law enforcement agencies like the F.B.I., the intelligence community, the news media, the opposition party and Congress. But to his great frustration, American institutions have mostly passed the stress test with flying colors.

“President Trump followed the electoral authoritarian script during his first year,” Levitsky and Ziblatt conclude. “He made efforts to capture the referees, sideline the key players who might halt him, and tilt the playing field. But the president has talked more than he has acted, and his most notorious threats have not been realized. … Little actual backsliding occurred in 2017.”

That seems right to me: The system worked.

And yet.

For all my confidence that our institutions will trump Trump, the chipping away at the integrity of our institutions and norms does worry me. Levitsky and Ziblatt warn of the unraveling of democratic norms — norms such as treating the other side as rivals rather than as enemies, condemning violence and bigotry, and so on. This unraveling was underway long before Trump (Newt Gingrich nudged it along in the 1990s), but Trump accelerated it.