WASHINGTON - Across the United States, nearly two-thirds of Democrats and 70 percent Americans said US President Joe Biden should step aside and let his party nominate a different candidate, according to a latest Associated Press (AP)-NORC Center for Public Affairs Research poll.

The AP-NORC poll, conducted from July 11-15, also found that only about three in 10 Democrats are extremely or very confident that he has the mental capability to serve effectively as president, down slightly from 40 percent in an AP-NORC poll in February.

Biden, who tested positive for COVID-19 while traveling Wednesday in Las Vegas, is facing growing calls from his own party to step aside, amid questioning about his age and mental fitness to do the job, after last month's dismal debate performance.

Prominent Democratic Representative Adam Schiff on Wednesday joined nearly 20 Congress members of his party to press Biden to withdraw from his reelection bid, as the Democratic National Committee grapples with uncertainties on the nomination process scheduled to complete by end of next month.

While the "choice to withdraw from the campaign is President Biden's alone", it is time for Biden "to pass the torch" and "secure his legacy of leadership" by allowing another Democrat to beat former president Donald Trump, Schiff said in a statement. The representative from California is close to former Speaker of the House Nancy Pelosi, making his call particularly notable.

Trump received enough delegate votes on Monday to officially become the Republican Party's nominee in the Nov 5 presidential election.

In response to Schiff's comments, the Biden campaign has pointed to what it called "extensive support" for him and his reelection bid from members of Congress in key swing states, as well as from the Congressional Black and Hispanic caucuses.

Biden is traveling in Nevada this week, and the campaign noted that he's been joined on the trip by "nearly a dozen" Congressional Black Caucus members, according to an AP report.

Meanwhile, the Democratic National Committee is pushing ahead with plans for a virtual vote to formally make Biden its nominee in the first week of August, before the Democratic convention scheduled for Aug 19-22 completes the nomination process.

While the exact date of the virtual vote remains undecided, a group of House Democrats have cautioned about swiftly nominating Biden as the party's pick for reelection, saying the virtual roll call would be a "terrible idea" to stifle debate about the party's nominee.

"It could deeply undermine the morale and unity of Democrats," according to a letter from the group obtained by the Associated Press.