US President Joe Biden has said he is “proud” to be the “first black woman…to serve with a black president” in his latest verbal blooper amid intense pressure to give up the presidential campaign.

In an interview with radio station WURD in Philadelphia, Biden muddled his words when referring to Vice President Kamala Harris.

Harris is considered the leading contender to take his place in the November 5 election if Biden were to drop out, sources have said.

But Biden has repeatedly said he will not step down from the race, despite growing calls for him to do so, following the first debate against former President Donald Trump.

In audio from the recent radio interview Biden said: “By the way, I’m proud to be, as I said, the first vice-president, first black woman… to serve with a black president.”

President Joe Biden and Vice President Kamala Harris watch July 4 fireworks from the White House balcony. Photo: Getty

As he scrambles to defuse a political crisis, Biden is due to take part in a high-stakes prime time interview on Saturday morning (AEDT) that will be closely watched for signs of whether his mental acuity is failing.

Biden will travel to Madison, Wisconsin, to rally Democratic voters.

During the trip he will be interviewed by ABC News, part of a flurry of events during the next week aimed at showing Americans he still has the stamina to run against Trump in the November 5 election.

If he mangles words or looks unfocused or confused, he will face renewed pressure to depart.

While Biden insists he is staying in the race and that his health is not faltering, he is under enormous pressure to step aside and open a path for his 59-year-old vice president.

Some donors are making their displeasure known loudly, halting funding or looking at possible Democratic alternatives.

In the latest blow, Abigail Disney, granddaughter of Walt Disney, will halt donations to the Democrats until Biden removes himself as a candidate, the heiress told CNBC.

Even some of Biden’s closest political allies, like former Speaker Nancy Pelosi, have raised questions about his health.

Several public opinion polls have shown Trump establishing a sizeable lead over the Democratic president since the debate, while a Reuters/Ipsos poll found one in three Democrats want Biden to quit the race.

The White House has blamed a cold for Biden’s shaky performance and Biden himself cited jet lag from back-to-back trips to Europe.

The ABC interview offers the likelihood of unscripted comments from Biden, who relies heavily on the use of a Teleprompter for his public remarks.

At Fourth of July holiday ceremonies on the White House South Lawn on Thursday night, Biden responded quickly when someone shouted at him to “keep up the fight”.

“You got me, man,” Biden responded. “I’m not going anywhere.”

Trump’s campaign and some of his allies have launched a pre-emptive political strike on Harris, moving swiftly to try to discredit her amid talk that she could eventually replace Biden as the Democrats’ nominee.

The Biden campaign showed no signs of slowing down, although the Trump team has overtaken it on fundraising.

The campaign announced it would spend $US50 million ($74 million) on a media blitz for the month of July, “including strategic investments around key events that draw in large and politically diverse audiences like the 2024 Olympic Games and the Republican National Convention”.

Trump, 78, who made multiple false statements from the debate stage in Atlanta, falsely claimed in a video that was circulated on social media that he had driven Biden out of the race.

He made disparaging comments about Harris in the same video, which the Trump campaign stood by.

-with AAP