But pollsters, Democrats and some Republicans on the ground say there are also unmistakable signs of more Democratic energy this year — even in “the belly of the beast,” as Ms. Simon put it — compared with 2016.

“Among those who didn’t show up in Pennsylvania, they were two-to-one Clinton supporters,” said Patrick Murray, director of the Monmouth University Polling Institute. “What we’re seeing right now is not the same lack of enthusiasm.”

“Many of these so-called shy Biden voters, who haven’t been talking about it before, are also the voters, the Democratic voters, who stayed home four years ago and now regret it,” he added.

A Monmouth poll of registered voters in Pennsylvania last week showed that Mr. Trump’s lead among white voters without a college degree had fallen from 22 points in early September to just nine points this month, suggesting that Mr. Biden was cutting into a demographic that is crucial to the president’s hopes of winning re-election. Multiple recent polls have found Mr. Trump’s standing with those voters down compared with his 2016 result in Pennsylvania.