Credit: Chang W. Lee/The New York Times

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.”

We tend to assume that the threat to democracies comes […]