In the race to the bottom for the title of worst American president, the same few sorry names appear at the end of almost every list, jockeying for last place. There’s Andrew Johnson, whose abysmal behavior during Reconstruction led to the first presidential impeachment. There’s Warren G. Harding, responsible for the Teapot Dome scandal. There’s hapless, hated Franklin Pierce; doomed, dead-after-32-days William Henry Harrison; and inevitably, James Buchanan, often considered worst of all because of how badly he bungled the lead-up to the Civil War.

But as historians consider the legacy of Donald J. Trump, it appears that even the woefully inadequate Buchanan has some serious competition for the spot at the bottom.

“Trump was the first president to be impeached twice and the first to stir up a mob to try to attack the Capitol and disrupt his successor from becoming president,” said Eric Rauchway, professor of history at the University of California, Davis. “These will definitely go down in history books, and they are not good.”

“I already feel that he is the worst,” said Ted Widmer, professor of history at the City University of New York, noting that as bad as Buchanan was — and he was very bad indeed — he was “not as aggressively bad as Trump.”

“Andrew Johnson and Nixon would be the two others in the worst category, and I think Trump has them beat pretty handily, too,” he added. “He has invented a whole new category, a subbasement that no one knew existed.”

Presidential ranking may be a water-cooler exercise for historians, but it is also an official institutional pursuit. The Siena College Research Institute regularly compiles ranked lists of all the American presidents, based on the composite views of scholars. So does C-SPAN.

Various polls periodically ask regular citizens to weigh in. And on Twitter last week, Chris Hayes of MSNBC took the presidential-ranking parlor game to his followers, asking them to list the “five worst presidents of all time.” (He put Mr. Trump as the second worst, just ahead of Andrew Johnson.)

Mr. Trump was a highly divisive president, of course, and one of the confounding things about him was how two people could look at his behavior and make completely different assessments.

But not so much anymore.

“I would say that before the election it depended on one’s political outlook,” with conservatives applauding his tax cuts, deregulation policies and judicial appointments, said William J. Cooper Jr., professor emeritus of history at Louisiana State University. “But from the election forward, I don’t see how anyone could feel that Trump’s behavior was anything but reprehensible or that he hasn’t completely destroyed any legacy he would have left.”