In this March 7, 2020, file photo Democratic presidential candidate former Vice President Joe Biden acknowledges the crowd during a campaign rally in Kansas City, Mo. In an effort to ease concerns about his age, the 77-year-old presumptive Democratic nominee has said he wouldn't seek reelection if his mental or physical health declined. He has also referred to himself as a “transition candidate," acting as a bridge to a younger generation of leadership. Credit: Charlie Riedel | AP

WASHINGTON — Joe Biden has longed to win the White House for more than three decades. If he finally makes it there after November’s election, he’s already talking about leaving.

In an effort to ease concerns about his age, the 77-year-old presumptive Democratic nominee has said he wouldn’t seek reelection if his mental or physical health declined. He has also referred to himself as a “transition candidate,” acting as a bridge to a younger generation of leadership.

Biden is rarely known for sticking to a script, and the comments are evidence of his candid style. But they’re also contributing to intense speculation about who is best positioned to lead the party after him.

“We do have a longer bench as Democrats, a younger bench in terms of elected leadership all across the country,” said Democratic strategist L. Joy Williams, chairwoman of Higher Heights PAC, which is dedicated to electing more women to national and statewide offices.

Biden has not ruled out running for a second term, in part because such an explicit pledge would immediately render him a lame duck in Washington, where political capital will be needed to manage the coronavirus recovery.

But the question of his long-term prospects looms over his candidacy, especially as he considers his options for vice president.

While someone like Elizabeth Warren could broaden Biden’s appeal among progressives, the 70-year-old Massachusetts senator wouldn’t be the face of a new generation many in the party are seeking. That might be an advantage for younger contenders, such as California Sen. Kamala Harris, 55, or Minnesota Sen. Amy Klobuchar, 59.

It’s an awkward dynamic for Biden, whose lead in the Democratic primary coincided with the onset of a pandemic, making it harder to establish himself as the party’s unquestioned leader. He can ill afford chatter about who might succeed him when he still faces a competitive race against President Donald Trump in the fall.

There are few historic precedents for a president opting against reelection. None has passed up a chance at a second term after just four years in the White House since shortly after Reconstruction. President Lyndon B. Johnson declined to seek a second, full term in 1968, but was already in office five years by then because of John F. Kennedy’s assassination.

There’s also no guarantee that Biden’s running mate will be the immediate president-in-waiting he envisions. Biden has pledged to pick a woman, but virtually no one under active consideration is likely to satisfy all Democrats. That raises the prospect of a primary battle in 2024 if he steps aside.

Republican pollster Chris Wilson said Biden might elevate several younger Democrats to Cabinet positions to deliberately set up “almost a hand-picked primary pool rather than a single candidate he tries to hand things off to.”

That, Wilson said, “would still be the kind of legacy-building move he seems to be interested in.”

Biden might also change his mind and decide to run for reelection if he unseats Trump. That still might not insulate him from a progressive primary challenge, though.

“Even if Biden wins and says he’s going to run in 2024, he’s absolutely going to be challenged from within the party,” said Eric Hauser, who was press secretary for Bill Bradley’s primary run in 2000 against Al Gore, who had been Bill Clinton’s vice president for eight years and was seen by many as his natural successor. “The left has felt like it got hoodwinked twice, in ’16 and now. They feel overlooked.”

Gilberto Hinojosa, chairman of the Texas Democratic Party, said Biden’s choice of running mate will have to fill twin roles. That person would need to continue to move the country away from the Trump era “if something were to happen while he’s still in office” or later “if Biden decides to retire and pass the baton.”

“I think the stakes are already really high, no matter how you look at it,” Hinojosa said.

Republicans could also face similar tumult. If Trump secures a second term, Vice President Mike Pence would seem to be a natural successor. But there are plenty of other Republicans with presidential ambitions who could be more attractive if Trump becomes unpopular at the end of a second term, which often happens to presidents after eight years in office.

George H.W. Bush is the only sitting vice president in modern history to be elected president. The only other examples are Martin Van Buren in 1836, and Thomas Jefferson and John Adams before that.

“Mike Pence would be inheriting eight years of a tsunami,” Hauser said of Trump’s legacy.

For all the speculation, if Biden is elected, he could decide to seek another term in 2024 in part because the lure of the White House is one of the greatest forces in politics — especially for someone ambitious enough to be on his third presidential bid since 1988.

“Once you’re president, it’s very tempting to keep power,” said Julian Zelizer, a history and public affairs professor at Princeton University who has written about single-term presidents. “It could very well be a genuine idea right now. But we just don’t see people relinquish power very easily.”