November 2016 was a bracing reminder that presidential elections are won or lost in the Electoral College, a fact Democrats may deplore but are powerless to change. Last week’s midterm election offered some important clues about Democrats’ prospects of capturing at least 270 electoral votes in 2020.

Two years ago, Hillary Clinton prevailed in 20 states and the District of Columbia with a total of 232 electoral votes. The midterm results offer no evidence that Donald Trump can peel away any of these states. If anything, he...