Democrats are learning the wrong lesson from Donald Trump

He ran as a moderate — and it worked.

Many progressives have what they believe to be a knock-down answer to nervous Nellies who fret that talking about desegregation busing, decriminalizing illegal entry into the United States, banning assault weapons, and replacing private health insurance will kill them at the polls in 2020: Donald Trump is president.

If Trump is president, the thinking goes, it’s the ultimate proof of “lol nothing matters” politics. And if anything does matter, it’s riling up your base to go to war, not trimming and...