The GOP long relied on wedge issues—God, guns, gays—to bury Democratic candidates running in competitive states and districts. But the country has evolved, and more voters are seeking candidates who embody diversity, openness and compassion. Louie Gohmert-style responses to the pandemic and stupefying denials of climate change have betrayed the GOP’s cultural aversion to science. The Black Lives Matter signs in windows across suburban the U.S. reveal what so many Americans think of Mr. Trump’s divisiveness. Set the underlying merits aside—these issues reveal a cultural shift that cuts against the Republican Party. Culture now works in the Democratic Party’s favor.

The issue isn’t whether college-educated suburban voters will help us beat Mr. Trump—they will. The challenge is to get them to stick with us beyond 2020. We don’t want these voters simply to “rent” the Democrat Party to remove Mr. Trump from the Oval Office. We want them to “buy” into our agenda so that they fuel legislative victories through the next decade on our core issues. To enact our agenda, we’ll need a coalition broad enough to withstand the electoral blowback that inevitably comes from leading big reforms. To do that, we need these voters to stay blue even in the lean years.

I understand the impulse to marginalize voters who weren’t with us in 2016—to castigate them for ignoring all the warning signs about Mr. Trump. I’m hardly known for being a “forgive and forget” kind of guy. But vengeance would be shortsighted and self-defeating.