Washington DC - Joe Biden's withdrawal upends what was already a White House race for the ages, but it will rattle Donald Trump, too, thus forcing him to recalibrate a campaign that was focused almost entirely on his former opponent.

For months Trump and his allies have been playing on concerns that the 81-year-old Biden may no longer be fit for office, gleefully sharing video clips and photos of every stammer, rhetorical gaffe, and red carpet stumble.

He was frequently a target of Republican mockery and scorn at the Republican National Convention in Milwaukee, where Trump's newly announced running mate JD Vance called him "Fake Scranton Joe."

But with the Democrat no longer in the race, the Trump operation finds itself pushed into a strategic pivot, forced now to shift its messaging to fit a contest with no incumbent and a yet-to-be-confirmed opponent, although Biden did endorse Vice President Kamala Harris on Sunday as he pulled out of the race.

Politicians Mike Pence demands US Senate reject Trump's appointment of RFK Jr. over abortion stance

"Biden withdrawing is bad news for Trump," said Henry Olsen, a senior fellow at the conservative Ethics and Public Policy Center think tank.

"Biden has the lowest job approval of any first-term president at this stage of his presidency in polling history and is also irretrievably burdened by his age. It's much better for Trump to run against him than any conceivable opponent."

Trumpworld had been playing down the chances of Biden withdrawing, but behind-the-scenes aides have been consumed by the contingencies, preparing a brutal assault on the veteran Democrat's heir apparent, Harris.

"Nothing fundamentally changes," said Jason Miller, one of Trump's closest advisors, during the Republican convention, saying that any "radical liberal" Democratic opponent, Harris or otherwise, shares "the responsibility for the failure of destroying our economy."

Harris, a 59-year-old former California senator, is by no means a shoo-in and could face competition from multiple leading Democrats in Congress and the nation's 23 Democratic governors.

Donald Trump Trump appoints his criminal defense attorneys to Justice Department positions

A campaign led by Harris and backed up by a moderate midwestern running mate could present the biggest danger though, helping turn out more women, who historically vote in greater numbers than men and are a weakness for Trump.

Harris would also give Democrats a chance to redefine the race at their convention in Chicago in August as a clash of cultures between a former prosecutor and a convicted felon.

And Harris ensures that abortion rights – one of her biggest domestic priorities and another vulnerability for Trump and the Republicans – remains a crucial election issue.

Trump's new, serious focus on the vice president was demonstrated by his bestowal of one of his famous nicknames, referring to her recently as "Laffin' Kamala Harris."

The vice president has the disadvantage of incumbency, though, meaning Trump can pin on her anything that was considered a weakness for Biden – including the border crisis, on which Harris led the administration's early efforts.

Several Democratic rising stars have been floated as alternatives, including governors Josh Shapiro, Gretchen Whitmer, and Gavin Newsom.

Newsom, of California – who has demonstrated a willingness to go toe-to-toe with top Republicans – would be unlikely to be intimidated by Trump's broadsides.

Whitmer was considered for the Biden ticket in 2020 and has since spearheaded a Democratic resurgence in Michigan, while Pennsylvania's Shapiro is another serious contender as chief executive of a critical swing state.

Whoever becomes the nominee, Trump will likely spend the rest of the campaign pushing them on their past defenses of Biden, accusing them of covering up the president's decline.

"The destruction and chaos created by the Biden administration isn't just on Crooked Joe – it's on the entire Democrat Party," the Trump team said in a recent edition of its daily circular.

"None have been worse than Kamala Harris, who has lied repeatedly – presumably putting her own position above the security of the American people."