It’s also possible that the Kavanaugh controversy is helping Republicans in Republican-leaning areas. It’s consistent with the available evidence, but this could also be noise considering the small number of polls since the hearing.

A highly polarized electorate hurts Democrats

The possibility that the Kavanaugh nomination is helping Republicans in Republican-leaning areas is important because the fight for control of both the House and the Senate will be determined largely in Republican-leaning areas. This simple fact has always been the G.O.P.’s biggest advantage. If the electorate is polarized along the lines of recent presidential elections, as it was during the Obama presidency, Republicans could hold down their losses considerably.

Democrats have been considered clear favorites in the fight for House control because polls and special election results have made it seem that the electorate wouldn’t be so polarized, allowing them to compete in many Republican-leaning districts. But if Democrats can’t break through and actually carry the many Republican-leaning districts they’ve put into play, Republicans could stay highly competitive in the fight for House control and even survive a wave election.

Today’s House map is so favorable to Republicans that based on recent presidential election results, even a 2006- or 2010-type wave — even a rerun of the highly polarized Virginia governor’s and state legislative races last November — would yield only around a net-27 seats for Democrats, by our estimates. Yes, that would be enough for a majority, but it would be close enough that it wouldn’t take too much luck for Republicans to hold on.

The 2006 election is a particularly telling example. Democrats picked up 31 seats, not much more than Democrats need now, with a set of opportunities fairly similar to what the Democrats have today. And the Democratic gain was padded by many victories against Republicans embroiled in scandal. Without those gains, the Democrats might not have picked up the number of seats that Democrats need this year.

There’s another reason 2006 is a troubling example for Democrats: The Republicans avoided a total rout by winning around 20 districts by less than four points. It’s not hard to imagine something like that happening again. In fact, Republicans have led in 12 Upshot/Siena polls by less than four points already.

For now, Democrats aren’t yet favored to win in most of the Republican-held seats they’ve put into play. The FiveThirtyEight election forecast, which gives the Democrats around a 75 percent chance to win the House with an average pickup of 34 seats, makes the Democrats outright favorites only in 218 districts — precisely the number needed for a majority. The Democrats are favored to win so many more races than they currently lead because there’s a lopsided number of Republican-tilting districts where the Democrats are highly competitive.

At the moment, not much separates a Democratic landslide from a seat-by-seat, piecemeal battle for control that lasts late on election night — or for days longer as mail ballots are counted in California and Washington.