Joe Biden’s withdrawal upends what was already a White House race for the ages, but it will rattle Donald Trump, too – 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 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.

“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 advisers, during the Republican convention.

He said 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 not a shoo-in, and could face competition from multiple leading Democrats in Congress and the nation’s 23 Democratic governors.

“I’m going to show up and I’m going to campaign – whether it’s him or somebody else,” Trump told a radio network in Virginia before Biden’s announcement, pointing to polls showing him doing as well or better against other Democrats.

Voters have been telling pollsters for months that they want younger political leaders, and a ticket topped by a relatively youthful swing state governor, for example, would be a threat to Trump, who would be 82 at the end of a second term.

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 on her of one of his famous nicknames, referring to her on social media recently as “Laffin’ Kamala Harris”.

It was the disastrous June presidential debate in Atlanta that proved the catalyst for Biden’s withdrawal.

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.

The Mercury