US President Joe Biden is facing battles on multiple fronts after testing positive for Covid-19 as Democrat heavyweights ramp up pressure on him to quit the election race.

A CNN report claims former House Speaker Nancy Pelosi privately told Biden that polling showed he could not defeat Donald Trump and he would destroy the Democrats’ chances of reclaiming office.

Politico says Pelosi told Biden in a candid conversation he was dragging down the Democrats and urged him step down as the party’s candidate.

Elsewhere, Senate Democratic Leader Chuck Schumer also went to see Biden at the weekend and told him in a “blunt conversation” that it would be best if he pulled out of the race, writes ABC in the US.

The mounting pushback from within the party comes as Biden was forced to cancel a planned speech in Las Vegas to isolate and keep up with other duties in Delaware, the White House said on Wednesday (US time).

He had developed respiratory symptoms, including a runny nose and cough, his doctor said.

Biden was meant to speak at an event in Las Vegas as part of an effort to rally Hispanic voters ahead of the November election.

Also on Wednesday, California Representative Adam Schiff joined the calls to urge Biden to withdraw from the presidential race.

Democrats are pushing ahead with plans to hold a virtual vote to formally make Biden their nominee in the first week of August before their national convention opens in person two weeks later.

Schiff is the 20th congressional Democrat, and the highest-profile one so far, to publicly urge Biden to withdraw from the presidential race. He is also a key Pelosi ally.

“A second Trump presidency will undermine the very foundation of our democracy, and I have serious concerns about whether the President can defeat Donald Trump in November,” Schiff, a California Democrat who is running for the Senate, was quoted by the Los Angeles Times on Thursday (AEST) as saying.

President Biden is vaccinated, boosted, and he is experiencing mild symptoms following a positive COVID-19 test.

He will be returning to Delaware where he will self-isolate and will continue to carry out all of his duties fully during that time.

A note from @POTUS‘ Doctor:…

— The White House (@WhiteHouse) July 17, 2024

In an interview on Wednesday, Biden changed his language about running for the November election. He said he would re-evaluate if a doctor told him directly that he had a medical condition that meant he should withdraw.

“If I had some medical condition that emerged, if somebody, if doctors came to me and said, you got this problem and that problem,” he said.

Biden said no doctors had yet said that. In February, White House physician Dr Kevin O’Connor wrote that Biden was “a healthy, active, robust, 81-year-old male who remains fit to successfully execute the duties of the presidency”.

Despite that, Democratic concern about the 81-year-old incumbent’s ability to beat Republican challenger Trump, or to serve another four years in office, surged after Biden’s halting June 27 debate performance.

Trump is in Milwaukee where his fellow Republicans are formally nominating him as candidate, days after he narrowly escaped an assassin’s bullet at a Pennsylvania campaign stop.

Late on Wednesday, Trump’s vice-presidential pick, JD Vance, introduced himself to Americans with a speech at the Republican National Convention.

“I grew up in Middletown, Ohio, a small town where people spoke their minds, built with their hands and loved their God, their family, their community and their country with their whole hearts,” he said.

“But it was also a place that had been cast aside and forgotten by America’s ruling class in Washington.”

He accused “career politicians” like Biden – who Vance noted has been in politics longer than he has been alive – of destroying communities like his with ill-fated trade policies and foreign wars.

“President Trump’s vision is so simple and yet so powerful,” he said. “We’re done, ladies and gentlemen, catering to Wall Street. We’ll commit to the working man.”

Nearly two-thirds of Democrat voters say Biden should step aside and let his party nominate a different candidate, according to an AP-NORC Centre for Public Affairs Research poll released on Wednesday. It undercuts Biden’s post-debate claim that “average Democrats” are still with him, even if some”big names” are turning on him.

“While the choice to withdraw from the campaign is President Biden’s alone, I believe it is time for him to pass the torch,” Schiff said.

“And in doing so, secure his legacy of leadership by allowing us to defeat Donald Trump in the upcoming election.”

Schiff’s call on Wednesday came after Schumer and House Democratic Leader Hakeem Jeffries encouraged the Democratic National Convention to delay for a week plans to hold a virtual vote to renominate Biden. It could have taken place as soon as Sunday, according to two people familiar with the situation.

Biden last tested positive for Covid-19 twice in mid-2022, when he had a primary case and a rebound case of the virus.

US health officials have reported recent upticks in emergency room visits and hospitalisations from the virus. There has also been a pronounced increase in positive test results in much of the US, particularly in the south-west.

-with AAP