Republican presidential candidate Nikki Haley has said it is unlikely Joe Biden would “make it” to the end of a second term were he to be re-elected to the White House.
Mr Biden, 80, announced he was running for re-election in 2024 on Tuesday.
He is already the oldest president in history. By the end of a second term, Mr Biden would be 86.
“He announced that he’s running again in 2024, and I think that we can all be very clear and say with a matter of fact that if you vote for Joe Biden you really are counting on a President Harris, because the idea that he would make it until 86 years old is not something that I think is likely,” Ms Haley said in a Fox News interview on Thursday.
She was referring to President Biden’s deputy, vice president Kamala Harris, who would take over the presidency if Mr Biden died in office. The vice president featured prominently in a video which launched Mr Biden’s re-election bid.
The White House issued a blunt response to Ms Haley’s comments, NBC News reported.
Deputy press secretary Andrew Bates said: “As you know, we don’t directly respond to campaigns from here. But honestly, I forgot she was running.”
Some Democratic voters have indicated they would prefer he not run, in part because of his age – concerns Mr Biden himself has called “totally legitimate”.
A recent poll from The Associated Press-NORC Centre for Public Affairs Research showed just 47% of Democrats said they wanted him to seek a second term, up from 37% in February.
“With regard to age, I can’t even say, I guess, how old I am. I can’t even say the number,” Mr Biden said on Wednesday at a joint news conference with South Korean president, Yoon Suk Yeol.
“It doesn’t register with me. I took a hard look at it before I decided to run. And I feel good.”
His video announcing his presidential bid shows him jogging in his suit.
Former US president Donald Trump launched his third bid for the presidency in November.
As it stands, analysts think a Biden-Trump 2024 election battle is likely, in a re-run of their contest for the White House in 2020.
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Ms Haley, who served as a United Nations ambassador under Mr Trump’s previous administration, became the second Republican candidate to throw her hat into the ring after her former boss.
The 51-year-old former South Carolina governor is seen as an outsider.
She announced her decision to run in February, saying it was time for a “new generation of leadership to rediscover fiscal responsibility, secure our border, and strengthen our country, our pride and our purpose”.
Former vice president Mike Pence is also reportedly exploring a possible challenge to Mr Trump for the Republican presidential nomination.
On Thursday, he appeared before a federal grand jury investigating Mr Trump’s role in efforts to overturn the 2020 election, according to reports.