NEW YORK — Many Republicans quickly called on President Joe Biden to resign and leave the White House after his announcement on Sunday that he would withdraw from the 2024 presidential race.

Republican leaders said that Biden's decision to step aside confirmed their view that he was not in cognitive shape to serve as president — an issue that has dogged the Democrat since his disastrous debate last month.

"If Joe Biden is not fit to run for president, he is not fit to serve as president," said House Speaker Mike Johnson, the most powerful Republican in congress.

"He must resign the office immediately. November 5 cannot arrive soon enough," he added, referring to Election Day.

Biden, in announcing that he was dropping out, said he would stay in office until the end of his term in January.

The White House a few hours later on Sunday reiterated he would not resign, stating: "He looks forward to finish his term and delivering more historic results for the American people."

Leading Republicans piled on with similar calls to resign throughout Sunday afternoon, as they also directed fresh attacks at Vice President Kamala Harris, who would move into the Oval Office should Biden resign. Biden has endorsed her to be the next Democratic nominee.

New York Representative Elise Stefanik, the Republican conference chair, made almost the exact same statement as Johnson's about Biden's ability to fulfill his presidential duties.

She closed her statement similarly as well: "He must immediately resign."

Biden's rival for president, Donald Trump, said the Democratic leader was "not fit to serve from the very beginning" in response to the announcement — though he did not call for the president to resign.

Senator Steve Daines of Montana, who chairs Senate Republicans' campaign arm, said that being president "is the hardest job in the world".

"And I no longer have confidence that Joe Biden can effectively execute his duties as Commander-in-Chief,” he said in a statement.

Another Republican senator, Markwayne Mullin of Oklahoma, went further and appeared to suggest that Biden should be forced from office by exercising the 25th Amendment of the US Constitution — a never-used method to replace the president if he cannot fulfil his duties.

Critics of Trump had called for using the amendment to remove him when he was in office.

Many in the political world had been expecting to Biden to drop out of the race.

His rambling, frequently incoherent answers in the June 27 debate with Trump had stunned the country and left people wondering if he could serve as president for another four years. While in speeches and interviews Biden often showed renewed vigour, he was also dogged by major stumbles and seeming memory problems.

Democrats in Congress, worried that his shakiness would hurt their chances at re-election, and major donors began to press for him to drop out, but they did not press for him to resign.

The last president to abandon his election campaign, Lyndon B. Johnson, also served out the remainder of his term. Like Biden, Johnson had said that giving up the race would allow him to focus on his presidential duties.

As the pressure on Biden has grown in recent weeks, Republicans became more vocal about a resignation.

Just hours before the president announced he was stepping aside, Trump's new running mate, Senator JD Vance of Ohio, said: "Not running for reelection would be a clear admission that President Trump was right all along about Biden not being mentally fit enough to serve as Commander-in-Chief. There is no middle ground."

"Joe Biden has been the worst President in my lifetime and Kamala Harris has been right there with him every step of the way," he added.

Biden has endorsed Ms Harris to take up the mantle of the presidential campaign, although the party will still have to formally approve its nominee.

"I am honored to have the President’s endorsement and my intention is to earn and win this nomination," Ms Harris said in a statement. "I will do everything in my power to unite the Democratic Party—and unite our nation—to defeat Donald Trump and his extreme Project 2025 agenda."

Republicans have reportedly prepared to attack Ms Harris's candidacy — as many believed she was the most likely successor.

Trump campaign sources have told US media outlets that they were readying attack ads and opposition research in case they faced her.

Most criticism centers on the vice president's lead role on immigration issues within the administration. Several speakers at the Republican convention last week portrayed Ms Harris as a failed "border czar".

Those attacks returned on Sunday.

Speaker Johnson called her "a completely inept border czar" and said she had been "a gleeful accomplice" in "the destruction of American sovereignty, security, and prosperity".

"She has known for as long as anyone of his incapacity to serve," he said, while also accusing her of being part of a political coverup of Biden's problems.

Republican Governor Greg Abbott of Texas, known for taking a hard line on immigration that has led to legal actions, also expressed concerns about Ms Harris becoming president.

"I think I will need to triple the border wall, razor wire barriers and National Guard on the border," he wrote on social media.

Donald Trump Jr, the former president's son, broadly said her policies would be no different than Biden's.

"Kamala Harris owns the entire left-wing policy record of Joe Biden. The only difference is that she is even more liberal and less competent than Joe, which is really saying something," he posted on X, formerly Twitter. — BBC