As Mr. Biden formally kicked off his campaign, he appeared at this point to be a virtual lock to win his party’s nomination. While many Democrats had hoped he would cede to a younger candidate, no formidable challenger for the nomination has emerged. Robert F. Kennedy Jr., a son of the iconic assassinated senator and a vocal critic of vaccines, and Marianne Williamson, the self-help author whose 2020 campaign fizzled before the first votes were cast, have announced long-shot bids but pose little evident threat to the incumbent president.
If Mr. Biden and Mr. Trump square off again next year, it would be the first time that the same nominees faced each other in consecutive presidential elections since 1956, when Dwight D. Eisenhower beat Adlai Stevenson for the second time. It would also be the first time a president was challenged by his predecessor since Theodore Roosevelt attempted a comeback in 1912 against his handpicked successor, William Howard Taft, in a three-way campaign won by Woodrow Wilson.
While Mr. Biden presides over a more unified party than his potential challenger does, many Democrats privately worry that the president may not be up to another campaign. His overall approval rating remains mired at just over 42 percent, according to an aggregation of polls by the political website FiveThirtyEight, lower than 10 of the last 13 presidents at this point in their terms.
While polls show that most Democrats have favorable opinions about Mr. Biden, a majority of them would still rather he not run again. In a survey by NBC News released this week, 70 percent of Americans, including 51 percent of Democrats, said he should not seek a second term. Seven out of 10 of those who did not want him to serve four more years cited his age as a factor.
Mr. Biden and Mr. Trump face a strikingly competitive race, with recent polls by Yahoo News, The Wall Street Journal and Morning Consult showing the president slightly ahead while surveys by The Economist and the Harvard University Center for American Political Studies find him trailing by several points. Mr. Biden faces similarly mixed results against Gov. Ron DeSantis of Florida, the strongest challenger to Mr. Trump for the Republican nomination.
Mr. Biden’s announcement represented the latest improbable chapter in a long life in public office, the fourth time he has thrown his hat in the presidential ring and presumably the last campaign of a half-century-plus career that began with his election to the New Castle County Council in 1970.
Over the course of 36 years in the Senate, eight years as vice president and campaigns for the White House in 1988, 2008 and 2020, Mr. Biden has become one of the most familiar faces in American life, known for his resilience in adversity as well as his habitual gaffes. And yet the avuncular, backslapping, work-across-the-aisle deal maker has struggled to translate decades of good will into the unifying presidency he promised.