Newly minted Republican vice presidential nominee JD Vance wrote in an X post Friday that if Democrats believe President Biden doesn’t have the mental acuity to continue to run for re-election, it would be difficult to argue he should remain president until next January.

"If Joe Biden doesn't have the cognitive function to run for re-election, then he certainly doesn't have the cognitive function to remain as Commander-In-Chief," Vance posted. "How can any Dem pushing him to drop out of the presidential race, argue in good faith that he should stay on as POTUS?"

Sen. Marco Rubio, R-Fla., echoed that sentiment to reporters at the Republican National Convention in Milwaukee Thursday. 

"The question is, if he's not going to be their nominee because he's not up to it, how can he be our president for the next six months?" Rubio said, according to Politico. "If there's something wrong with you that doesn't allow you to run for president, how can you still be there as president? If they're going to remove him as nominee, they’ve got to remove him as president, and that's really bad for our country."

FIVE MORE HOUSE DEMOCRATS CALL ON BIDEN TO DROP OUT, THIRD US SENATOR

JD Vance and Biden

Sen. JD Vance, who became former President Trump’s running mate last week, said if Democrats believe President Biden doesn’t have the mental acuity to continue to run for re-election, it would be hard to argue he should remain president until next January. (Getty)

Polling guru and FiveThirtyEight founder Nate Silver earlier this month wrote on X he also believes Biden should "transition the presidency to [Vice President Kamala] Harris within 30-60 days, but I’m there now. Something is clearly wrong here."

Rep. Marie Gluesenkamp Perez, D-Wash., has also called on Biden to step down. 

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"Americans deserve to feel their president is fit enough to do the job," she told KGW-TV. "The crisis of confidence in the president’s leadership needs to come to an end. The president should do what he knows is right for the country and put the national interest first."

Marco Rubio at the RNC

Sen. Marco Rubio also suggested that Biden should resign. (Victor J. Blue/Bloomberg via Getty Images)

Increasing numbers of Democrats have called on Biden to exit the White House race after his disastrous debate performance late last month, fearing he would lose to Trump. 

Historian Alexis Coe wrote in a Rolling Stone op-ed Friday that Biden should resign to "preserve his legacy."

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Presidential historian Allan Lichtman, who has accurately predicted nine of the last 10 elections, has also said that if Biden were to drop out of the race, handing over the presidency to Harris as she replaces him on the ticket would be the Democrats' best chance of keeping the White House in November. 

Biden and his campaign have said the president is "in it to win it," although reports have said he is reconsidering that amid increasing calls to drop out.