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 .

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

Biden鈥檚 decision to 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 .

鈥淚t shakes things up entirely,鈥� said Dan Pfeiffer, a former adviser to President Barack Obama. 鈥淚t turns everything on its head.鈥�

The crumbling of Biden鈥檚 Democratic reelection effort, which began with 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鈥檚 team claimed it was , and Republicans stepped up their criticism during last week in Milwaukee.

However, Trump himself expressed disappointment that 鈥渨e have to start all over again鈥� with the campaign. He mused on Truth Social, his social media platform, that Republicans should be 鈥渞eimbursed for fraud鈥� for all the money they鈥檝e 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, 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 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鈥檚 status as a convicted felon after being 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鈥檚 campaign as his running mate.

鈥淚n every possible way, this is the anti-Trump,鈥� the narrator said. 鈥淪o if that鈥檚 what you鈥檙e looking for in your next president, there鈥檚 really only one 鈥� Kamala.鈥�

The leaders of Trump鈥檚 campaign dismissed the dramatic change, saying, 鈥淜amala Harris is just as much of (a) joke as Biden is.鈥�

鈥淭hey own each other鈥檚 records, and there is no distance between the two,鈥� 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 鈥渂order czar鈥� and blamed her for unauthorized crossings.

鈥淭hey鈥檙e still going to run the Gotham City playbook with an added dose of racism and sexism,鈥� said Cornell Belcher, a Democratic pollster. 鈥淎nd let鈥檚 not pretend that those things don鈥檛 matter, because they do.鈥�

However, Belcher said, Harris鈥� 鈥淴 factor鈥� is her potential appeal to a diversifying electorate.

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

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

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

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

鈥淪he is someone who really values young voices in general,鈥� he said.

At last week鈥檚 Republican National Convention, Trump pollster and senior advisor Tony Fabrizio said the campaign was 鈥�100% ready鈥� to take on Harris. He noted speakers at the event often referred to the 鈥淏iden-Harris鈥� 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.

鈥淣ow 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,鈥� Trump wrote on Truth Social.

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