The White House says President Joe Biden is “staying in the race” and has pushed back at a report that he may not be able to salvage his candidacy after a disastrous debate performance.

The New York Times reported that Biden told an ally he was weighing up whether to continue his re-election bid, and that he needed to quickly convince the public.

But the White House rubbished that report as “absolutely false”.

At a briefing on Thursday morning (AEDT), Press Secretary Karine Jean-Pierre told reporters that Biden “is not dropping out”.

“The president is clear-eyed and he is staying in the race,” she said.

Meanwhile CNN reported that Biden held a call with his campaign team and vowed he was “not leaving”.

“Let me say this as clearly as I possibly can and as simply and straightforward as I can: I am running. I’m the nominee of the Democratic Party,” CNN reported the source.

“No one’s pushing me out. I’m not leaving. I’m in this race to the end and we’re going to win because when Democrats unite, we always win.”

He told the all-staff call that the past few days had been “tough”, wrote CNN.

White House: ‘No excuses’

The White House briefing was dominated by questions from journalists on Biden’s health, his sharpness, whether he took a daily nap and if he was physically capable of another term.

Jean-Pierre said Biden was not “making excuses” for his debate performance, but explained that he had been suffering from a cold and the effects of two “rigorous” foreign trips.

“It’s the jet lag and it’s also the cold — it’s the two things — you all heard it in his voice,” she said.

“This is not an excuse. You asked for an explanation and we are giving it, but it’s not an excuse.

“It wasn’t his best night, it wasn’t a great debate,” she conceded.

When asked if Biden was as sharp now as when he was elected, Jean-Pierre said: “This is a president who is strong and resolute”.

“He is as sharp as ever, as I have known him to be.”

The New York Times had earlier reported that Biden told an unnamed  ally he was weighing whether to continue in the race.

The text of the report did not include any reference to those comments.

CNN later reported a similar conversation with an unnamed ally, who was quoted as saying that Biden is “not oblivious”.

In its report, CNN said Biden had privately acknowledged to an ally that the next stretch was critical to whether he can save his re-election bid.

“The polls are plummeting, the fundraising is drying up, and the interviews are going badly. He’s not oblivious,” CNN quoted the ally as saying.

Biden’s campaign has held damage control calls with donors, and is planning to hold a meeting with Democratic governors later on Wednesday.

Meanwhile, Vice President Kamala Harris has been the White House’s first line of defence following the dismal debate showing.

She conceded to a few dozen donors in San Francisco, the president’s performance was “not his finest hour.”

However, she said, “the outcome of this election cannot be determined by one day in June.”

The moment could hardly be more delicate for the 59-year-old Harris, who is the first woman, Black person and person of South Asian descent to serve as vice president.

Although some Democrats have pointed to her as a logical successor if Biden steps aside, others are drawing up wish lists of potential replacements that don’t include her on the ticket at all.

Depending on how the coming days and weeks unfold, her work on Biden’s behalf could lead to a political dead end, or secure her future within the party by providing fresh opportunities to prove her doubters wrong.

They were scheduled to have one of their occasional private lunches on Wednesday, and the vice president was belatedly added to the schedule for his Fourth of July celebration at the White House on Thursday.

Biden and Trump neck-and-neck: Poll

But there’s a sign the contest may be close, with Biden pulling even with his Republican challenger Donald Trump, according to a new Reuters/Ipsos poll.

Biden and Trump each had 40 per cent support among registered voters in the two-day poll that concluded on Tuesday.

A previous Reuters/Ipsos poll conducted on June 11-12 showed Trump with a marginal two percentage-point lead, 41 per cent to 39 per cent.

The two faced off in a televised debate last Thursday, where Biden stammered throughout and failed to challenge Trump’s attacks.

The new Reuters/Ipsos poll also showed that, following the debate, about one-in-three Democrats think Biden should drop out of the race, something he has pledged not to do.

The poll, which gathered responses online and nationwide from 1070 US adults, had a 3.5 percentage point margin of error for registered voters, many of whom remain on the fence with about four months left before the November 5 election.

One in five registered voters said that they were not sure for whom to vote, that they would pick a different candidate or that they would not vote at all.

The latest poll did not include a question on support for independent candidate Robert Kennedy Jr.

The June poll found 10 per cent of registered voters would back him if he appeared on the ballot.

While nationwide surveys give important signals on American support for political candidates, just a handful of competitive states typically tilt the balance in the US electoral college, which ultimately decides who wins a presidential election.

The latest Reuters/Ipsos poll found 83 per cent of Democrats and 97 per cent of Republicans agreed with a statement that “Biden stumbled and appeared to show his age” in the debate.

Only 58 per cent of Democrats and 11 per cent of Republicans had the same assessment of Trump’s debate performance.

Trump, 78, in May became the first US president to be convicted of a crime, whether in office or after leaving the White House.

He is scheduled to be sentenced in September and faces potential prison time after a jury found him guilty of 34 charges stemming from a hush money payment to an adult film actress before the 2016 presidential election.

-with AP