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.

But in an era characterized by declining rates of split-ticket voting and an enormous, growing urban vs. rural divide (by my math, in 2016, Donald Trump carried 76 percent of counties with a Cracker Barrel but 22 percent of counties with a Whole Foods Market), holdouts like Mr. Tester and Mr. Katko could be more the exception than the rule.