After Biden announced that he would not seek reelection, Republican lawmakers called for his resignation.

"If Biden is not suitable for running for president, he is not suitable for being president," said House Speaker Mike Johnson in a post on X after Biden announced his decision. "He must resign immediately."

Other Republican lawmakers, including House Republican Conference Chairman Elise Stefanik and the National Republican Senatorial Committee, also expressed support for Johnson's remarks.

In a statement, former President Trump's campaign team said: "Biden is a national security threat with severely diminished cognitive abilities, and is an obvious danger to every man, woman, and child in our country. So the question for Vice President Kamala Harris is simple: Does she think Americans will be safe with Biden in the White House for another six months after he drops out of the race due to illness?"

Some Republican lawmakers even raised the possibility of invoking the 25th Amendment, which sets out the order of succession when a president is unable to perform his or her duties. South Carolina State Representative Nancy Mace posted on X that she planned to introduce a resolution on Monday calling on Harris to invoke the 25th Amendment.

When former Vice President Mike Pence and the Trump Cabinet were in their final days in office, the Democratic-controlled House of Representatives passed a measure in 2021 calling on them to invoke the 25th Amendment. Pence refused to do so.

Biden, who is 81 years old, performed poorly in the June debate, stumbling over his words and sounding hoarse, which exacerbated concerns about his age and fueled calls for him to drop out of the race. But the President did not mention these issues in his statement, nor did he mention his age. He made it clear that he will serve out the remaining six months of his term.

In his letter to X, Biden wrote: "I believe that it is in the best interests of my party and my country for me to step back from the campaign and focus on fulfilling my duties as President during the remainder of my term."

After his annual physical examination in February of this year, White House physician Kevin O'Connor said that Biden "is a healthy, vigorous, 81-year-old male who is still fit to successfully execute the duties of the Presidency." Biden also underwent a brief medical check-up after the June 27th debate, and he told Democratic governors in the following week that the doctor told him he was in good health.

After testing positive for the coronavirus last Wednesday, Biden is currently undergoing isolation and treatment in Delaware. O'Connor said on Sunday that the President's symptoms have "markedly improved." In his latest update, he wrote: "The President continues to be treated without difficulty and will continue to take PAXLOVID as planned. He is continuing to perform all of the duties of the Presidency."

Republican lawmakers began calling on Biden to resign before he announced that he would not seek reelection.

"If Biden ends his reelection campaign, how can he continue to serve as President?" Senator JD Vance, Trump's running mate, said on X Sunday morning. "Not seeking reelection is tantamount to admitting that President Trump has been right all along about Biden's inability to lead as commander-in-chief."

According to The New York Times, the American Tradition Partnership and other conservatives are preparing to launch a legal battle over changes to the Democratic presidential ticket, and the Trump campaign is also looking for ways to attack Vice President Harris, especially if she becomes the nominee.

"We officially call on Biden to resign from the presidency, as his continued presence in the War Room poses a threat to national security," said Bernie Moreno, who is running for Senate in Ohio, in a statement. "There is no doubt that Senator (Sherrod) Brown knew about Biden's mental decline, but he covered it up."

NRSC Executive Director Jason Thielman told Axios: "If Biden is deemed unfit to meet the rigorous demands of being a presidential candidate, then logically he is even less suitable for the higher-stakes, higher-impact role of the presidency."

NRCC Communications Director Jack Pandol said: "Democrats can't have it both ways—they need to answer whether Biden should resign, or if he's still capable of controlling the nuclear button."

But not all Republican lawmakers are calling for Biden's resignation. Senate Minority Leader Mitch McConnell and House Majority Leader Steve Scalise did not say that Biden should not complete his remaining term in their remarks.