“The advantage that [Harris] has is that she is the newer, fresher face,” Dent said. “Biden was struggling in Pennsylvania for the same reasons he was struggling with voters in all of these other states: they thought he was too damn old. That was true in Pennsylvania and it was true all over the country.”

Ed Rendell, the former Democratic governor of Pennsylvania and a former chair of the Democratic National Committee, said Harris had “heightened enthusiasm”, which would bring out more voters than Biden.

“But what percentage of the vote is she going to get?” he said. “Will she persuade the voters who haven’t made up their mind? That remains to be seen.”

Democrats and non-partisan analysts said that to win Pennsylvania, Harris would need to run up her margins in the traditional Democratic strongholds of Philadelphia and its suburbs while stemming losses in rural parts.

But she has faced doubts about her shifting stance on fracking for shale gas, an important part of western Pennsylvania’s economy — opposing the practice in 2019 but now supporting it.

Her reversal on shale, like her comments this week that US Steel should remain “American owned and American operated”, were seen as calculated moves to appeal to blue-collar and labour union voters.

Others insist Pennsylvania’s vote will ultimately be decided by voters in the middle of the income bracket and the ideological spectrum.

“It is important to talk about the construction jobs and the union jobs and the blue-collar jobs,” said Michael LaRosa, a former Biden administration official and a Pennsylvania native. “But where Democrats tend to falter is by losing the people in the middle, the people who are not in construction and blue-collar jobs or union jobs, who are not wealthy and who make up the vast majority of the suburbs of Philadelphia, Pittsburgh and Scranton and Reading. It’s really important that you speak to [them] too.”

Additional reporting by Oliver Roeder in New York