In 2018, Democrats picked up four House seats in Pennsylvania, largely in suburban areas, and Mr. Trump is struggling in those regions. Mr. Biden has made them a primary focus, as he did on Saturday, when he spoke in Dallas Township, a suburb of Wilkes-Barre, hammering Mr. Trump for his handling of the coronavirus crisis.

But polls show that Mr. Biden has yet to fully capitalize on the softening of Mr. Trump’s support. Averaging the results of the most recent Times/Siena and Morning Call/Muhlenberg polls, in the Philadelphia suburbs Mr. Trump is polling nine points behind the share he received there in 2016 exit polls, but Mr. Biden is only three points ahead of Mrs. Clinton’s total among this group.

Troubling for Mr. Biden in a different way is the fact that he has not yet matched Mrs. Clinton’s share of support in Philadelphia proper. Averaging the results of the two recent polls, he has the backing of 73 percent of Philadelphia voters, down from 83 percent for Mrs. Clinton in 2016. According to the Times/Siena poll, Mr. Trump was supported by 24 percent of Philadelphians, nine points ahead of his exit poll numbers in 2016.