NEW YORK (AP) — Donald Trump's campaign has spent the last year-and-a-half viciously 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 Biden might exit the race, readying a bevy of attacks against Vice President Kamala Harris that they unleashed as soon as Biden made his stunning announcement Sunday that he would step aside. Biden soon after endorsed Harris, who was quickly winning 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 speakers at the event often referred to the "Biden-Harris" administration in their speeches and said the campaign had prepared anti-Harris videos to swap in just in case Biden stepped down sooner.

Still, the shakeup less than four months before Election Day lays out new challenges for Trump's team, which had until recently been focused on contrasting the former president's vigor and mental acuity with Biden's. Trump will now face a new, yet-to-be-determined opponent at a time when voters have made clear that they are frustrated by their current choices and desperate for new, younger options.

Though Trump aides had wanted Biden to remain in the race, they have argued a campaign against Harris — who they believe is the most likely Democratic nominee — wouldn't be all that different from their race against Biden.

They will try to tie her to what they see as the Biden administration's failures, saying Harris is complicit in everything that occurred under Biden's watch. That's particularly true when it comes to the handling of the Southern border. Harris had been tapped to lead the administration's response to the border crisis.

In a statement Sunday responding to the news, Trump campaign senior adviser Chris LaCivita and fellow campaign chief Susie Wiles railed against Harris, insisting she "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 coverup of Biden's deteriorating health, believing voters have lost trust in Democrats and the press for failing to shed light on Biden's weaknesses sooner. And they will 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 Biden — still running against him, in a way — by arguing that if he is not fit to run than 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," LaCivita and 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.

Trump's campaign had tried to goad Biden to stay in the race, including by casting Democrats' efforts to replace him as a "coup."

But they began ramping up their attacks on Harris right after the debate last month.

In a post celebrating Independence Day, Trump singled out Harris on his Truth Social site, calling her his "potentially new Democrat Challenger" and giving her a new derisive nickname: "Laffin' Kamala Harris."

While Trump insisted publicly that he still believed Biden would be the ultimate nominee, he was captured in an expletive-laced video saying he thought she was "going to be better" as a rival.

"She's so bad. She's so pathetic," he said.

At his rally in Michigan Saturday night, Trump polled the crowd, asking if they'd prefer that he face Biden or Harris. The crowd erupted into cheers when Trump mentioned Biden. Harris' name was met with boos.

He also continued to mock Harris' laugh, and called her "nuts" and "crazy."

When the news finally came, Republicans were ready.

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

Trump's chief super PAC, Maga Inc., also released a new ad that tried to blame Harris for Biden's policies.

"They created the mess. They — no, Kamala — owns this failed record," says the narrator.

The chaos now engulfing the Democratic Party as it scrambles to find a new candidate comes just days after the conclusion of the Republican National Convention, where Republicans rallied around Trump, presenting a united front, after he narrowly survived an attempted assassination at a rally in Pennsylvania.

Republican National Committee chairman Michael Whatley cast the unfolding situation as "quite a split screen."

"We are a completely unified party," Whatley said in an interview on Fox News Channel, while "the Democrats are in free fall."

Whatley said Republicans were not going to change their plans despite the Democratic shakeup.

"President Trump is going to run his race and whether it's Kamala Harris or anybody else, they are going to run on the exact same failed agenda that Joe Biden has been running over the last four years," he said.

It remains unclear, however, how a new candidate atop the Democratic ticket will change the dynamics of the race.

Polling has shown Harris' favorability ratings are similar to those of Trump and Biden. A June AP-NORC poll found about 4 in 10 Americans have a favorable opinion of her, though the share of those who have unfavorable opinion was slightly lower than for Trump and Biden.

At 59 years old, Harris would be a marked generational contrast to 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.

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

Harris has also been the Biden administration's leading voice on abortion rights, an animating issue for Democrats since the overturning of Roe v. Wade that could again motivate turnout this fall.

Facing the prospect of debating a former prosecutor, Trump on Sunday called for a change of venue to friendlier territory for the next debate, saying it should be moderated by Fox News instead of the previously-agreed-to ABC.

Speaking at an event hosted by the University of Chicago Institute of Politics and The Cook Political Report, Fabrizio, the pollster, said last week that Harris is fairly undefined for a vice president.

"The one thing voters know about her is her laugh," Fabrizio said. "And that cuts both ways for her."

Republicans have also hinted at the possibility that they they could try to take legal action to keep Biden on the ballot.

But Edward B. Foley, a law professor who heads The Ohio State University's election law center, said political parties control their nomination processes and any legal challenge by Republicans would be unlikely to succeed.

"I just don't see how the Republican Party or anyone associated with the Republican Party would have any standing to bring any litigation in connection with this,' he said. Unlike general elections, in primaries, "voters provide input, but they don't control the decision."

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Associated Press writer Christina Almeida Cassidy contributed to this report from Atlanta.