Mr. Biden led Mr. Trump, 49 percent to 40 percent, among likely Pennsylvania voters in the poll, which was conducted Friday through Sunday. On Saturday, Mr. Trump nominated Judge Amy Coney Barrett for the court seat and immediately flew to Pennsylvania to whip up support that night, speaking before a giant screen with the words “FILL THAT SEAT!”

But that prospect — which the Trump campaign is counting on to shift the election dynamic away from the president’s handling of the coronavirus pandemic — has not reset the race there. On the contrary, 51 percent of voters in the passionately divided battleground state said they trusted Mr. Biden more to pick the next justice, whereas 44 percent said that about Mr. Trump…

Mr. Biden has not trailed in a public poll of Pennsylvania since June. But his lead narrowed over the summer as the state trended back toward tossup status, evoking memories of Mr. Trump’s slim victory there in 2016. The president has virtually no path to a second term without Pennsylvania’s 20 electoral votes. Mr. Biden could afford to lose the state, but only if he runs the table in other northern battlegrounds that Democrats narrowly lost in 2016 and picks up Arizona.