Demographics is not enough.

For years now, it’s been an article of faith among Democrats that the future belongs to them, thanks to the country’s changing demographic mix. The rising percentage of voters who are women, Americans of color and especially Latinos were always about to turn the country deep “blue.”

I never believed this — largely because I have been hearing it since 1971. That was the year the 26th Amendment passed, lowering the voting age to 18. Democrats had already been the dominant political party since the 1930s, and now with young people getting the vote, a permanent Democratic majority was guaranteed, right?

The future failed to arrive on time again this fall. Democrats lost all over America, and they lost big, by much wider margins than predicted. They lost statewide races in the Midwest where Democrats have won repeatedly in presidential elections for more than 20 years. They lost in races against radical right-wing Republicans they might have been expected to defeat, like Sam Brownback in Kansas and Paul R. LePage in Maine.

Nor was this month’s election an anomaly. It was the third disastrous midterm for Democrats in the past 20 years. The party suffered similar or even worse losses at all levels of government in 1994 and 2010, along with a lesser catastrophe in 2002. It now holds fewer elected offices at both the federal and state level than it has at any time since the 1920s...