WASHINGTON (AP) — For the past year, the presidential campaign seemed destined to be a monotonous slog featuring two candidates, President Joe Biden and former President Donald Trump, who voters didn't really want.

But that all changed on a quiet Sunday afternoon just 107 days before the election.

Biden's decision to drop out of the race and endorse Vice President Kamala Harris as his successor resets the campaign with a swiftness that is unparalleled in modern American politics.

Once a contest between two older white men, the election will likely force Trump to contend with the much younger Harris, who was consolidating support among Democrats and would be the first woman of color atop a major party's ticket.

"It shakes things up entirely," said Dan Pfeiffer, a former adviser to President Barack Obama. "It turns everything on its head."

The crumbling of Biden's Democratic reelection effort, which began with his shaky debate performance against the Republican former president last month, has left both parties scrambling. Although no one has stepped forward to challenge Harris for the Democratic nomination, she still faces the unprecedented challenge of taking over a campaign only four weeks before the party gathers in Chicago for its convention.

At the same time, Trump must pivot his focus to Harris after designing his campaign for a rematch with Biden. Trump's team claimed it was prepared to run against the vice president, and Republicans stepped up their criticism during the party's convention last week in Milwaukee.

However, Trump himself expressed disappointment that "we have to start all over again" with the campaign. He mused on Truth Social, his social media platform, that Republicans should be "reimbursed for fraud" for all the money they've spent running against Biden.

The shakeup in the presidential race came after an extraordinarily tumultuous month in American politics, starting with an unusually early debate between Biden and Trump.

The June 27 showdown catalyzed concerns that Biden, 81, was too old for a second term.

As Democrats pushed to dislodge him from the top of the ticket, Trump survived an assassination attempt on July 13 during a campaign rally in Butler, Pennsylvania.

Just a week later, Biden bowed to pressure within his party to step aside. He swiftly endorsed Harris, who is the first woman, Black person and person of South Asian descent to serve as vice president.

The breakneck developments left the political world gasping for breath as everyone tried to reorient themselves to a new reality.

Trump, who is 78, will almost certainly be the oldest candidate on the ballot after spending months battering Biden over his age. Harris is only 59, giving her a claim to representing generational change that Biden could never fulfill.

In addition, Harris is a former prosecutor, providing a fresh opportunity to assail Trump's status as a convicted felon after being found guilty in a hush money trial earlier this year.

She is also the daughter of immigrants, raised by a Jamaican father and an Indian mother, a background that heightens the contrast with Trump, who has used racist, sexist and nativist rhetoric.

Harris tried to summarize their differences in a campaign advertisement five years ago, when she was seeking the Democratic nomination before dropping out and joining Biden's campaign as his running mate.

"In every possible way, this is the anti-Trump," the narrator said. "So if that's what you're looking for in your next president, there's really only one — Kamala."

The leaders of Trump's campaign dismissed the dramatic change, saying, "Kamala Harris is just as much of (a) joke as Biden is."

"They own each other's records, and there is no distance between the two," said a statement from senior advisers Chris LaCivita and Susie Wiles.

Immigration will remain a key line of attack against Democrats, especially because Harris was tasked by Biden to work on migration issues early in the administration. Republicans claimed she was appointed as a "border czar" and blamed her for unauthorized crossings.

"They're still going to run the Gotham City playbook with an added dose of racism and sexism," said Cornell Belcher, a Democratic pollster. "And let's not pretend that those things don't matter, because they do."

However, Belcher said, Harris' "X factor" is her potential appeal to a diversifying electorate.

"When you look at her, she is the Democrats' best chance right now to re-engage and energize that coalition of younger, browner voters," he said.

Democrats were eager to turn a political weakness that hounded Biden — his age — into an attack on Trump.

"This will probably boil down to Donald Trump, who is the oldest nominee in history, against Kamala Harris," said Rep. Maxwell Frost, a 27-year-old Democrat from Florida who has worked to reach young voters for Biden's campaign.

Frost, who endorsed Harris, pointed to the vice president's work on gun violence protection as an issue that could engage young voters and said she "will be able to win back a lot of the youth vote."

"She is someone who really values young voices in general," he said.

At last week's Republican National Convention, Trump pollster and senior advisor Tony Fabrizio said the campaign was "100% ready" to take on Harris. 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.

However, Trump seemed to have some doubts. After Biden dropped out of the race, Trump suggested he was having second thoughts about participating in another debate hosted by ABC News on Sept. 10.

"Now that Joe has, not surprisingly, has quit the race, I think the Debate, with whomever the Radical Left Democrats choose, should be held on FoxNews, rather than very biased ABC," Trump wrote on Truth Social.

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Associated Press writers Stephen Groves, Jill Colvin and Michelle Price contributed to this report.