In Pennsylvania, Joe Biden Finds Support Where He Most Needs It

“I like his sincerity, his personality, his experience,” he said. He added, “I think he cares.”

Mike Mikus, a political consultant in Pittsburgh who also advises unions, recalled that Mr. Biden was greeted warmly at last year’s Labor Day parade in the city. “It’s one of the largest Labor Day parades in the country,” he said. “They were all yelling, Please run, Joe!”

Mr. Sanders, too, is competing for white voters without a college degree. He recently defended his policies on “Medicare for all” and free public college, and his opposition to trade deals — which have pushed the party leftward — during a blitz of Midwest states.

“The reason we are visiting these states is pretty simple: Donald Trump won them two and a half years ago, and we’re not going to let him win them in 2020,” he told a crowd of thousands in Pittsburgh.

Mr. Trump has made clear he will fight hard to carry Pennsylvania again in 2020, and he remains broadly popular with working-class white voters who believe he generally reflects their interests and outlook better than many Democratic leaders.

But some Republicans already see the state as vulnerable: The G.O.P. was battered in the 2018 midterms, with a net loss of three House seats, and party officials are concerned that Democrats will nominate a candidate who will have broader appeal than Hillary Clinton did in 2016.

In the Muhlenberg College poll, Mr. Biden’s greatest weakness appeared to be with younger voters. He was the choice of only 5 percent of Democrats under 30, compared with 40 percent for Mr. Sanders.

One of those voters is Ian Gallagher, a 24-year-old casino employee from Northampton. The argument that Mr. Biden’s center-left politics makes him more electable “doesn’t do it for me,” he said. “We need way more left than center-left. Center-left was Obama, and that didn’t work out.” He favors an Elizabeth Warren-Bernie Sanders ticket, with Ms. Warren at the top.