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. —STEVEN LEVITSKY and DANIEL ZIBLATT