And Republicans have a good chance to flip two seats themselves — Pennsylvania’s 14th and Minnesota’s Eighth — which would put the Democrats at a net gain of 13.

Four of the 15 seats poised to flip to the Democrats are in Pennsylvania, where Democrats benefit from a new congressional map drawn by the state’s Supreme Court. Five others are in open races where Republicans retired and Democrats have strong recruits.

Then there are six Republican incumbents who are already thought to be at a big disadvantage. Five of them are in relatively white suburban districts: Colorado’s Sixth, Minnesota’s Second, Minnesota’s Third, Kansas’ Third and Virginia’s 10th. A sixth, Iowa’s First, is in a mostly white working-class but traditionally Democratic district where Barack Obama won easily in 2012.

We can’t say that Democrats have truly locked down these races, though the party’s candidates led by at least seven points in each of the New York Times Upshot/Siena polls of these districts. All are rated as “lean Democratic” by the Cook Political Report, and there’s varying talk that the G.O.P. will withdraw resources in these races.

But there’s still vigorous spending here, and polling is often sparse or out of date. It wouldn’t be a surprise if Republicans ultimately pulled it out in at least one of them in the final stretch.