Former Vice President Kamala Harris said it was a mistake not to question President Joseph R. Biden Jr.’s insistence on running for re-election last year despite his advanced age.

In a new book, she writes that it was “recklessness” that led her and other Democrats to cede the choice “to an individual’s ego.”

Ms. Harris’s assertion places her alongside scores of other figures in the Democratic Party who, since President Trump took office, have publicly admitted that Mr. Biden’s re-election effort at age 81 was a mistake. Long before Mr. Biden’s disastrous debate performance against Mr. Trump in June 2024, Democrats had privately questioned whether he should run again.

“During all those months of growing panic, should I have told Joe to consider not running? Perhaps,” she wrote in an excerpt from her forthcoming book published on Wednesday in The Atlantic. “But the American people had chosen him before in the same matchup. Maybe he was right to believe that they would do so again.”

The former vice president described herself as “in the worst position to make the case that he should drop out” of the 2024 race because she believed Mr. Biden would view such an entreaty as disloyal. At the time, she and others extended “grace” in allowing Mr. Biden and Jill Biden, the first lady, to make the decision on their own. But now, she wrote, she has come to believe it was a mistake not to speak up.

“In retrospect, I think it was recklessness,” she wrote. “The stakes were simply too high. This wasn’t a choice that should have been left to an individual’s ego, an individual’s ambition. It should have been more than a personal decision.”