In the follow-up to their 2018 bestseller “How Democracies Die,” authors Daniel Ziblatt and Steven Levitsky write about three rules that political parties must follow: accept the results of fair elections, reject the use of violence to gain power and break ties to extremists.

In the aftermath of the 2020 election, they write, only one U.S. political party “violated all three.”

Saturday marks the third anniversary of the Jan. 6, 2021, attack on the Capitol, and Donald Trump, the former president, is far-and-away the leading Republican candidate in 2024. He still refuses to acknowledge his earlier loss to President Joe Biden. Far from rejecting the rioters, he has suggested he would pardon some of those who have been convicted of violent crimes. Rather than distance himself from extremists, he welcomes them at his rallies and calls them patriots.