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.

After two months largely consumed by the war in the Middle East, Mr. Biden is heading back to the campaign trail this week with three fund-raisers in the Boston area on Tuesday, another one in Washington on Wednesday and a three-day trip to Nevada and California this weekend. Aides have dismissed concerns from worried Democrats by arguing that once the choice is clear, wavering Democratic and independent voters will return to Mr. Biden even if concerned about his age, rather than allow Mr. Trump back into office.

The president assailed Mr. Trump on Tuesday for a radical agenda if he returns to office. Among other things, Mr. Trump or his allies have talked about prosecuting political foes and former Trump aides who have criticized him; eliminating civil service protection for many government officials to make them more personally loyal to him; setting up detention camps for illegal immigrants; backing off NATO treaty commitments; and sending the military into the streets to quell protests.

“Trump’s not even hiding the ball anymore,” Mr. Biden said at another of the fund-raisers on Tuesday. “He’s telling us what he’s going to do. He’s making no bones about it.”

Mr. Biden noted that Mr. Trump has vowed “retribution” against perceived enemies, promised to repeal the Affordable Care Act, supported further restrictions on abortion and threatened to root out “vermin” in Washington in language that the president compared to that used in Germany in the 1930s during the rise of Adolf Hitler and the Nazis.

“He didn’t show up at my inauguration, which I can’t say was a disappointment,” the president said at his last reception of the day. “My guess,” he added to applause, “is he won’t show up at my next inauguration.”