From a lengthy profile in New York Times Magazine:
Biden and his advisers are convinced that the general election will mostly be a referendum on Trump and his fitness for office. “This is really about character and values as opposed to issues and ideology,” says Mike Donilon, Biden’s chief strategist.
He acknowledges that Hillary Clinton tried and failed to make Trump’s suitability the pivotal question of the 2016 election.
The difference this time, he says, is that Trump is now president and has demonstrated his inadequacy. Biden made a similar point. “Even when he was running,” Biden told me, “I don’t think anybody thought he would be as bad as he is.”