Republicans will be forced to recalibrate their campaigns from coast to coast in the wake of President Biden’s announcement that he would not run for reelection, while newly energized Democrats wasted no time rallying to Vice President Harris as their potential standard-bearer. The vice president, due to her position, is not as well-defined as Mr. Biden is, and could reshape the campaign in the coming weeks. 

Immediately after suspending his campaign, Mr. Biden announced he would support his vice president as the nominee this year, and the party, in large part, quickly coalesced around her. 

The chairwoman of the Progressive Caucus, Congresswoman Pramila Jayapal and the chairwoman of the more centrist New Democrats, Congresswoman Annie Kuster, both endorsed Ms. Harris on Sunday. Two swing-district members who called on Mr. Biden to drop out, Congresswoman Hillary Scholten and Congressman Mike Levin, both endorsed the vice president. Senators Kaine, Warren, and Baldwin, too, announced their support for Ms. Harris. President Clinton and Secretary Clinton came out to support her, as well. 

Republicans were clearly incensed by Mr. Biden’s decision on Sunday, saying that he was betraying the democratic process by invalidating primary voters’ ballots. Some even called on him to step down as president. 

“If Joe Biden is not fit to run for President, he is not fit to serve as President. He must resign the office immediately,” Speaker Johnson wrote on X.

“After lecturing others about democracy, they just forced Joe Biden off the ticket — trashing the primary choice of 14 million of their own voters,” says the House Majority Leader, Congressman Steve Scalise. 

The problem for Republicans now is that they spent four valuable days at their convention making no case against the vice president herself. They sometimes mentioned her in speeches, referring to her as the “border czar,” though it only happened a handful of times. 

The Trump campaign has already deployed a 30-second advertisement airing in some swing states mentioning Ms. Harris’ role as the so-called “border czar” and accusing her of covering up the president’s lack of fitness for office. 

Mr. Johnson on Sunday hinted that Republicans could get behind legal challenges to a last minute candidate swap, including making it more difficult for a new Democrat nominee to gain ballot access in swing states, though he made no specific mention of what statutes would prevent another nominee from leading the ticket. 

“It would be wrong, and I think unlawful, in accordance with some of these states’ rules for a handful of people to go in a back room and switch it out because …they don’t like the candidate any longer,” Mr. Johnson told ABC News. “That’s not how this is supposed to work. So I think they would run into some legal impediments in at least a few of these jurisdictions.”

Mr Johnson has made clear in the past that he would have loved to see Mr. Biden atop the ticket in order to help his Republican House colleagues and candidates run against the deeply unpopular president.

“I think there’s a lot of hand-wringing because a lot of these people, particularly those who are in close races to get reelected, they know there’s a real serious domino effect here and they’re worried about their own political careers,” Mr. Johnson said at a Politico event during the Republican National Convention, barely containing his joy. 

The legal challenges to which Mr. Johnson has alluded would be spearheaded by the Heritage Foundation, which already has a plan in place to go into courthouses across the country should Mr. Biden be replaced by a new nominee. 

The conservative think tank has already drafted a memo stating they would file suit in Wisconsin, Georgia, and Nevada due to state laws that allegedly limits a party’s ability to swap out a presidential nominee. 

“Some states allow withdrawal before the 74th day before an election, and failure to adhere to these timelines can result in the candidate’s name remaining on the ballot,” the foundation says. “Likewise, the rationale for withdrawal (death, medical, or other) can be outcome determinative. Some states, like South Carolina, do not allow withdrawal for political reasons.”

It wasn’t just GOP elected officials and campaign staff who wanted the president to stay on the ballot. An ABC News poll released Sunday found that more Republican voters than Democratic voters wanted Mr. Biden to remain the Democratic nominee. For Republicans, 44 percent wanted the Democrats to stay the course, while just 39 percent of Democrats said Mr. Biden should soldier on. 

The poll respondents said that Mr. Biden was the most dissatisfying potential Democratic nominee, by far. In total, 55 percent of all voters said they were disappointed Mr. Biden was on the ballot while 34 percent were satisfied. Vice President Harris fares much better than her boss, with only 46 percent say they would be dissatisfied with her as the nominee, compared to 39 percent who would be satisfied.