An absurd debate is taking place in the Democratic Party: To beat Donald Trump in 2020, do we motivate our base or do we try appealing to the Obama-Trump voters who won him the Rust Belt? To win a presidential election, you don’t choose between your core supporters and swing voters. You have to appeal to both, and the way to do it is to win the economic narrative.

The Democratic nominee will have to lay out an agenda to provide all Americans the opportunity to earn a good life. In 2016 Mr. Trump won in part because his economic...