Donald Trump’s political campaign has spent the last year and a half attacking Joe Biden, ridiculing his policies, mocking his fumbles and relishing a rematch they felt they were winning.

But they have also spent weeks preparing for the possibility that Mr Biden might exit the race, readying a bevy of attacks against Vice President Kamala Harris that they unleashed as soon as Mr Biden made the announcement on Sunday that he would step aside.

Mr Biden soon after endorsed Ms Harris, who quickly won support from Democrats to be the party’s nominee.

“Rest assured, we are 100% ready,” Trump pollster and senior adviser Tony Fabrizio said at last week’s Republican National Convention.

He noted that speakers at the event often referred to the “Biden-Harris” administration in their speeches and that the campaign had prepared anti-Harris videos in case Mr Biden stepped down sooner.

Still, the shake-up less than four months before election day lays out new challenges for Mr Trump’s team, which had until recently been focused on contrasting the former president’s vigour and mental acuity with Mr Biden’s.

Though Trump aides had wanted Mr Biden to remain in the race, they have argued that a campaign against Ms Harris — who they believe is the most likely Democratic nominee — would not be all that different from facing Mr Biden.

They will try to tie her to what they see as the Biden administration’s failures, saying Ms Harris is complicit in everything that occurred under Mr Biden’s watch.

In a statement on Sunday responding to the news, Trump campaign senior adviser Chris LaCivita and fellow campaign chief Susie Wiles said Ms Harris “will be even WORSE for the people of our Nation than Joe Biden”.

“They own each other’s records, and there is no distance between the two,” they said.

They will also continue to accuse her of being part of a cover-up of Mr Biden’s deteriorating health, believing voters have lost trust in Democrats and the press for failing to shed light on Mr Biden’s weaknesses sooner.

They will also attack her record in California, where she served as district attorney and attorney general before being elected to the Senate.

They have also made clear that they plan to continue to hammer Mr Biden — still running against him, in a way — by arguing that if he is not fit to run then he is not fit to complete his term, and should resign as well.

“Joe Biden cannot take himself out of a campaign for President because he is too mentally incompetent and still remain in the White House,” Mr LaCivita and Ms Wiles wrote in their memo.

“Biden is a national security threat in great cognitive decline and a clear and present danger to every man, woman, and child in our country.”

At his rally in Michigan on Saturday night, Mr Trump asking the crowd if they would prefer that he face Mr Biden or Ms Harris. The crowd erupted into cheers when Mr Trump mentioned Mr Biden. Ms Harris’s name was met with boos.

He also continued to mock Ms Harris’s laugh, and called her “nuts” and “crazy”.

When the news finally came, Republicans were ready.

Less than an hour after Mr Biden’s announcement, Mr Trump’s campaign was filling its social media channels with clips of past statements by Ms Harris that could turn off some voters, including one of her supporting a ban on plastic straws.

Polling has shown Ms Harris’s favourability ratings are similar to those of Mr Trump and Mr Biden.

A June AP-NorcC poll found about four in 10 Americans had a favourable opinion of her.

At 59 years old, Ms Harris would be a marked generational contrast to Mr Trump, who turned 78 last month. She would also be the first black woman and the first person of south Asian descent to serve as president — a potentially barrier-breaking candidacy that could draw new support from women, minority voters and younger voters.

Mr Trump has a long history of making insulting comments about women and people of colour, something Ms Harris is likely to highlight on the debate stage and campaign trail.

Ms Harris has also been the Biden administration’s leading voice on abortion rights, an animating issue for Democrats since the overturning of Roe vs Wade that could again motivate voter turnout.