The larger Hillary Clinton's polling margin over Donald J. Trump grows, the louder the question becomes: Is control of the House of Representatives really in play?

Among House strategists in both parties, the answer remains the same as it has been all year: not yet. Democrats must gain 30 seats to capture a majority. That requires sweeping nearly all Republican-held seats in which they nurse even small hopes of winning.

Yet the interplay between the presidential race and others on the ballot has made those small hopes bigger. So far, to the relief of Speaker Paul Ryan, pollsters don't see evidence of a broad Democratic wave. Mr. Trump's candidacy has veered so far outside traditional political norms that it hasn't changed how voters regard most conventional Republicans.

Their prospects nevertheless remain connected, at least loosely. Robert Erikson, a Columbia University scholar of links between the presidential contest and down-ballot races, notes two competing influences. The coattails of popular White House candidates lift House candidates of their party by drawing like-minded voters to the polls. Because of party polarization, those backing Democratic presidential candidates typically back Democrats for the House and Senate, too.