Biden Says ‘I’m Not Sure I’d Be Running’ if Not for Trump

President Biden suggested on Tuesday that he might have been content to serve only a single term if his predecessor, former President Donald J. Trump, were not attempting to recapture the White House.

At a campaign fund-raiser in the Boston area, Mr. Biden presented his decision to run for re-election as driven largely by his determination to defeat Mr. Trump a second time and prevent him from returning to power. Mr. Biden has at times portrayed a second term for Mr. Trump as an existential threat to American democracy.

“If Trump wasn’t running, I’m not sure I’d be running,” he told donors at the Weston, Mass., home of Alan Solomont, a longtime Democratic financial backer who served as ambassador to Spain. “But we cannot let him win.”

The president’s remark came at a time when polls show that most Democrats would prefer someone other than Mr. Biden, who turned 81 last month, to represent the party in next year’s election. A survey by CNN in August found that 67 percent of Democrats and independents who lean Democratic wanted another nominee, and 70 percent listed Mr. Biden’s age, health, mental competence or ability to handle the job as their main concern about him.

Although he described himself as “a bridge” to the next generation during his 2020 campaign, a comment that some interpreted as a hint that he would serve only one term, Mr. Biden has concluded that he is best positioned to beat Mr. Trump again, justifying a re-election campaign. He faces only long-shot challengers in the Democratic primaries in the form of Representative Dean Phillips of Minnesota and Marianne Williamson, the author.

Mr. Trump, who is 77 and has demonstrated his own cognitive issues lately, has outpaced his rivals for the Republican nomination by double digits in the polls and appears poised to steamroller to his third general election. That is despite four criminal indictments on 91 felony counts of illegally trying to overthrow an election, endangering national security and other charges. Despite his political liabilities, surveys show he is either tied with Mr. Biden or leading slightly both nationally and in the battleground states that will decide the Electoral College.