The proper way to view the 2018 midterms might not be as one event, but as two very different elections playing out at once. It’s almost Mars vs. Venus: The Senate hinges on red, rural states where Democrats are on defense. But the House will be decided by swing, suburban seats where Republicans are highly vulnerable.

These radically different maps could produce dramatically divergent results — and make Congress even more toxic and adversarial in 2019.

It's hard to believe, but true: If every state’s and district’s election results on Nov. 6 were a uniform eight-point swing in the Democrats’ direction from the 2016 presidential result, Democrats would gain 44 House seats — almost twice the 23 they need to control the chamber. But with that same eight-point swing, the party would lose four Senate seats, leaving them six seats short of a majority.

A bifurcation that extreme is highly unlikely, because a handful of incumbents are personally popular enough to defy their constituents’ normal partisan preferences. For example, Senator Jon Tester, Democrat of Montana, and Representative John Katko, Republican of New York’s 24th District, an upstate area around Syracuse, have cultivated strong, independent brands: In Mr. Tester’s case, as a farmer who butchers his own meat; in Mr. Katko’s case, as a tough-on-gangs prosecutor. The Cook Political Report rates both of them as favorites for re-election.