Who Might Replace Biden on the Top of the Ticket?

President Biden’s stumbling performance in the debate against former President Donald J. Trump has some Democrats raising the possibility of nominating an alternative candidate and mulling over a roster of names. High on the list is, of course, Vice President Kamala Harris, whose status as Mr. Biden’s running mate might make her an easy candidate for delegates to turn to during a difficult moment. But a crop of Democratic governors and other figures are often mentioned, too.

A candidate switch would most likely require Mr. Biden to step out of the race, something his campaign says he has no intention of doing. And the risks are real. Some of the highest-profile potential stand-ins listed below have never endured the vetting and road test of a presidential race. There is a long list of candidates who looked great on paper and withered on the campaign trail.

“It’s not as easy as it sounds,” said Barbara Boxer, the former senator from California. “Being vetted for president is like no other vetting. We don’t know how these people would do.”

Here are a few of the contenders being discussed:

Kamala Harris

Vice President Harris, a former prosecutor and senator of California, has at times struggled to define her role at Mr. Biden’s side. Initially charged with addressing polarizing and intractable issues like illegal migration and voting rights, she has been viewed by Democratic donors and supporters of Mr. Biden as a potential political liability. Though those concerns have eased, she has been weighed down by low approval ratings that are barely higher than the president’s.

Still, Ms. Harris has for months stumped for the president as one of his main campaign surrogates. She has recently become the White House’s lead voice as a defender of abortion rights. In March, she met with abortion providers at a clinic in St. Paul, Minn., in what is believed to be the first visit by a president or vice president to an abortion clinic. And Ms. Harris, the nation’s first Black vice president, has worked to shore up Mr. Biden’s vulnerabilities with Black and young voters.