President Biden testified positive for COVID on Wednesday, a new setback for the 81-year-old embattled president on the same day that a prominent Democratic lawmaker joined the chorus for Mr. Biden to quit his reelection campaign, and polling showed more voters abandoning him. 

The news of Mr. Biden’s illness surfaced late in the day when he canceled the second of two events in Las Vegas. White House physician Dr. Kevin O’Connor said the president has a runny nose, a cough and “general malaise.”

“He felt okay for his first event of the day, but given that he was not feeling better, point of care testing for COVID-19 was conducted, and the results were positive for the COVID-19 virus,” the doctor said in a statement. “Given this, the president will be self-isolating in accordance with CDC guidance.”

Mr. Biden will return overnight to his beach home in Rehoboth Beach, Delaware. Press secretary Karine Jean-Pierre said the president, who is vaccinated and boosted, “will continue to carry out all of his duties fully.”

The timing and optics of the diagnosis couldn’t be worse for Mr. Biden, who is combatting the idea that he is too frail to be president. He had complained of a cold during his poor debate performance on June 27. 

The president was forced off the campaign trail on the same day that Rep. Adam Schiff, the Democratic nominee for a Senate seat in California, became the latest lawmaker calling on him to drop out of the presidential race.

Mr. Schiff, speaking to the Los Angeles Times, said he’s worried about Mr. Biden’s ability to defeat former President Donald Trump.

“Our nation is at a crossroads,” Mr. Schiff said. “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.”

Mr. Schiff said it’s time for Mr. Biden “to pass the torch” and “secure his legacy of leadership” by letting someone else lead the Democratic ticket.

Mr. Schiff is the 21st congressional Democrat to call on Mr. Biden to exit the presidential race, but the first to do so since Saturday’s assassination attempt on Mr. Trump. His decision to reopen the pressure campaign could lead to more Democrats publicly urging Mr. Biden to step aside and allow the party to nominate a new leader. 

Democrats are increasingly concerned about Mr. Biden’s advanced age of 81 and cognitive abilities, particularly after his lousy debate performance on June 27.

Yet Mr. Schiff is a particularly high-profile voice. His decision to go public with his concerns is notable given his close relationship with former Speaker Nancy Pelosi, who is reportedly working behind the scenes to pressure Mr. Biden to drop out. 

Publicly, Ms. Pelosi has said she will support whatever Mr. Biden decides but made clear she believes the president still has time to reconsider running. 

The president wrapped up a two-day campaign swing through Nevada on Wednesday, and was scheduled to return after midnight to his beach home in Rehoboth Beach, Delaware.

There was more ominous news for Mr. Biden in polling released Wednesday showing that his campaign his hemorrhaging support. CNN, citing internal Democratic polling, reported that Mr. Biden has lost ground to Mr. Trump in 14 key states from Nevada to Maine since the debate last month.

And nearly two-thirds of Democratic voters said Mr. Biden should bow out of the race and let the party nominate a replacement, the poll from the AP-NORC Center of Public Affairs found.

The survey, conducted July 11-15 among 1,253 voters, showed that 65% of those surveyed take that stand. Nearly half of Democratic voters were “not at all confident” about Mr. Biden’s mental capacity to be president for a second term.

Before Mr. Biden’s disastrous debate against Mr. Trump, 42% of those surveyed were very or somewhat satisfied with the president as the Democratic nominee. That figure dropped to 37% in the latest poll.

Vice President Kamala Harris, who would be the most likely choice of the party to replace Mr. Biden at the top of the ticket, urged supporters Wednesday not to give up on the November election. At a campaign event in Michigan, Ms. Harris urged voters to stay engaged despite the temptation to become jaded about U.S. politics.

“This is not a time to throw up our hands, this is a time to roll up our sleeves,” Ms. Harris said. “We should not become dispirited.”

Amid the renewed turmoil, Democrats said they will delay a planned online vote to nominate Mr. Biden ahead of the party’s Aug. 19 convention in Chicago. Democratic National Convention officials had planned to begin a multi-day, online vote to nominate Mr. Biden beginning as early as Sunday, but will delay starting the vote until the first week of August. 

The delay came amid protests from House Democrats who urged DNC officials to ditch the online nominating plan in favor of the in-person format. A letter signed by roughly 20 Democrats demanded the DNC halt the plan to nominate Mr. Biden virtually, calling it “a terrible idea,” and “unnecessary and unprecedented.” 

The new date would still lock in Mr. Biden, 81, as the party’s nominee ahead of the convention, which would break with the tradition of having the 4,000 delegates nominate their candidate in person. 

Mr. Schiff was a key leader in investigations and impeachment efforts against Mr. Trump, leading to high-profile acrimony with the GOP nominee. He is parlaying that status into a Senate run this year for the seat opened up by Sen. Dianne Feinstein, a Democrat who died last year.

As the front-runner in his Senate race against Republican nominee Steve Garvey, Mr. Schiff said the choice whether to run “is President Biden’s alone.” He also said he would support Mr. Biden if he decides to stay as the nominee.

Mr. Garvey, a well-known former baseball player for the Los Angeles Dodgers, announced a day earlier that he out-raised Mr. Schiff in the second quarter, $5.4 million to $4.2 million.

The developments in the Democratic Party caught the eye of Mr. Trump, who is in Milwaukee for the GOP convention after surviving an assassination attempt on Saturday.

“From a political standpoint, an unprecedented event is happening — The Dems are trying, again, to get Biden off the ballot!!!” Mr. Trump wrote on Truth Social.

— Lindsey McPherson, Alex Miller and Susan Ferrechio contributed to this report.

• Tom Howell Jr. can be reached at thowell@washingtontimes.com.