‘Boooooo!” That’s how California Democrats, gathered for their state party convention in San Francisco this weekend, received two of their 2020 presidential aspirants. Perhaps John Delaney and John Hickenlooper expected the jeering, given their remarks. Still, they deserve credit for telling their copartisans hard truths.

Mr. Delaney, a former Congressman from Maryland, is polling at 0.5% in the Real Clear Politics average. He told the convention that he supports universal health care, universal prekindergarten and free community...