Indeed, for all of the talk about 2016 fallback plans, Mrs. Clinton remains the front-runner for the nomination, and her comments on Tuesday in an interview with ABC News and in a follow-up message to supporters — saying “I’m sorry” for relying on private email for government business as secretary of state and calling that practice “a mistake” — may help assuage concerns among some Democrats about her candidacy.

But the chatter could continue if Mrs. Clinton does not quickly regain her footing.

“You have Democrats beginning to panic about the one thing that a lot of them never worried about, which was Clinton’s electability in the general election,” said Robert Shrum, a veteran strategist who was a senior adviser to Mr. Gore and Mr. Kerry during their presidential runs. “You still have to think of her as the odds-on favorite for the Democratic nomination. But the challenge she faces in the general election is both the trust problem and the likability problem.”

Mr. Shrum recalled how Mr. Gore’s likability suffered in the 2000 campaign, most memorably when he was ridiculed for supposedly having claimed he invented the Internet — “but not his fundamental trustworthiness, because there’s an assumption that all politicians exaggerate.”

Several Democrats said that Mr. Biden and Mr. Kerry were especially well positioned to enter the race late, given their experience, party support, fund-raising networks and name recognition.

“Biden is the kind of highly respected, well-known figure that, if he were to jump in the race during the primaries in an emergency kind of way, he could attract a lot of voters very quickly,” said Jaime Harrison, the chairman of the South Carolina Democratic Party.

Democrats close to Mr. Kerry said he would not consider the white-knight scenario unless drafted — and, indeed, add that he would probably laugh at it. In 2003, they recalled, when Mr. Kerry was struggling in the polls, there was chatter that Gen. Wesley Clark would ride to the rescue so former Vermont Gov. Howard Dean would not capture the nomination. (General Clark never became a strong contender.)