This is partly because Mr. Biden has run such a low-profile campaign during the pandemic. He has had few public appearances and news conferences, which can provide the unscripted moments opponents can use to shape the public’s perception of a candidate…

“It is going to be more difficult for the Trump campaign to go after a man who really is a centrist, has dealings with people across the aisle and knows how to talk to people who disagree with him,” said Priscilla Southwell, a professor emerita of political science at the University of Oregon. “And 2020 is a different kind of year. Donald Trump can appeal to his core by being negative, but it’s such a difficult time for everybody. I don’t think negativity is going to sell as well as it used to.”…

Mr. Trump had been adept at this. But the kind of attacks that seemed so effective when he was a new-to-politics outsider in 2016 also appear to have less resonance coming from inside the White House. Four years of tweets by Mr. Trump have numbed many voters.

“It’s almost self-defeating,” said Ron Christie, a Republican who was a senior adviser to President George W. Bush and Vice President Dick Cheney. “People are exhausted. The president, with every tweet, every insult, will move himself out of favor with the demographic that he needs the most, which is the independent.”