Democrats have a single goal when it comes to Donald Trump: impeachment. Their strategy is likewise clear: Resist! What no one seems to ask is whether resistance is really the best path to the House majority Democrats would need to pass articles of impeachment.

Democrats do have a few things going for them this year. On average, the party that holds the White House loses 30 seats or so in midterm elections—and the GOP has only a 24-seat majority. Moreover, 35 House Republicans are leaving their seats, more than twice the...