More than a dozen Republicans called on US President Joe Biden to step down on Sunday after he ended his re-election bid, saying that his unwillingness to continue to campaign raises questions about his ability to continue to govern.

House of Representatives Speaker Mike Johnson, US Senator JD Vance – the just-nominated running mate of Donald Trump in the November 5 election – and other lawmakers urged the 81-year-old incumbent to step down from office.

Biden insisted in his statement that he would finish his term, which ends January 20, 2025, and fellow Democrats blasted the calls as "ridiculous."

"If Joe Biden is not fit to run for president, he is not fit to serve as President. He must resign the office immediately. November 5 cannot arrive soon enough," said Johnson, who is next in line to the presidency after Vice President Kamala Harris.

Biden's fellow Democrats for weeks had called on him to end his campaign after a disastrous June 27 debate against former President Trump in which Biden at times struggled to finish his thoughts.

"If Joe Biden ends his reelection campaign, how can he justify remaining President?" Vance tweeted.

Others, including Republican Senator Markwayne Mullin, called on Biden's Cabinet to remove him from office by invoking the 25th Amendment to the US Constitution. Biden's Cabinet, composed of officials hand-picked by the president, is highly unlikely to do so.

Asked about the resignation calls, Democratic Senator Chris Coons of Delaware, a close associate of Biden, said, "I think that is ridiculous." (Reuters)