Jackie Zimmers, a stay-at-home mother from nearby Butler County, who was sporting a “Biden for President” button on her baby carrier, predicted “he would get some of the Democrats back” that the party lost in 2016.

Some Democrats, however, are concerned that the 76-year-old Mr. Biden has ample vulnerabilities, many related to his long record as a lawmaker. Anita Hill said last week that she was still dissatisfied with how Mr. Biden, then a senator, treated her when she testified before his committee during Justice Clarence Thomas’s 1991 confirmation hearings.

Mr. Biden made no mention of the matter during his Monday speech, but in an interview set to air Tuesday on ABC’s “Good Morning America,” he sought to bring closure to the issue by saying, “I take responsibility that she did not get treated well.”

Mr. Biden’s debut here as a candidate Monday represented something of a soft entry into the race for him, and was suffused with political and personal symbolism.

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Western Pennsylvania was once filled with reliably Democratic voters, many of them union members, but the state tipped to Mr. Trump three years ago in part because many working-class white voters in this region abandoned their ancestral party. No Democratic nominee has won the White House without carrying Pennsylvania since 1948, and the promise of Mr. Biden’s candidacy is that his appeal with this bloc of the electorate could swing the state, along with other Midwest battlegrounds, back from the Republicans.

Democrats barely broke a sweat in retaining the governorship and a Senate seat last year and picked up three House seats, all in the heavily suburban, southeastern part of the state where Mr. Trump’s popularity has plummeted.

Mr. Biden, as he rarely fails to note, spent his early childhood years in Scranton and has long trumpeted his status as this state’s “third senator.” He is particularly close to union officials here, having jogged his way through Pittsburgh’s Labor Day parade in the past, including in 2015 when he was considering a presidential run.