Polling is broken: The golden age, when everyone had a phone at home and picked it up, is over. It is now far more difficult and expensive to poll, and surveys in the same state at the same time often produce very different results. There’s also the “shy Trump” factor: It’s real, but how big is it?

Campaigns increasingly rely on modeling as a substitute for polling, using big data to define each voter’s predisposition and then run simulations. It sounds so scientifically precise, but I shared a stage the day before the 2016 election with a co-chairman of Hillary Clinton’s super PAC, who said she’d won 93% of some 60,000 simulations they’d just run and would be elected president on the morrow.

The hostility of the press toward Mr. Trump and its benign approach to Mr. Biden is also a difference. The national press corps is generally antagonistic to, or at least deeply wary of, any GOP presidential candidate. Still, the animus of reporters and their outlets toward Mr. Trump is gobsmacking. We’re in an era of an openly partisan press, dominated by the left. That makes two against one: Mr. Biden and the media against Mr. Trump.