In August we presented a number of congressional districts that could contribute to a wave election in 2016. The ensuing weeks have brought an increasingly close presidential contest and the congressional battleground has crystallized. Democrats need to win 30 seats in November for a new House majority. Right now, our outlook for Democratic gains this year remains unchanged, between 10 and 20 seats.

In the summer, Donald Trump became the Republican nominee, and his presence has figured to impact the House races in a dramatic and dichotomous way. Put very simply, Mr. Trump is likely to help Republicans in economically downscale and rural districts, while suburban voters (especially women) are likely to generate gains for Democrats in several states. With the election only five weeks away, other fundamentals of the battle for House control remain the same: Republican gains have reached a ceiling; undoing the impact of redistricting; higher turnout in a presidential election; open-seat contests.

The Landscape Favors Democrats

Republicans started the cycle far more vulnerable than Democrats, due to their 68-seat net gain over the 2010, 2012, and 2014 cycles. In that period, they amassed their largest House majority in almost a century. Democrats have been relishing this scenario all year. With very few battlegrounds to defend, they are almost assured a pick-up of at least a handful of seats. The outlook is so favorable, it is possible that every Democratic incumbent running for reelection might win this November. At this time only three are in jeopardy.

Favorable Redistricting Decisions

Court decisions related to redistricting in Florida and Virginia have resulted in new district maps in those states. The decisions have benefited Democrats, potentially delivering a net gain of at least three seats. We previously noted that Democrats will lose in Florida’s 2nd while picking up seats in Florida’s 10th, Florida’s 13th, and Virginia’s 4th Districts. It now appears that Florida’s 7th District could also change hands as a result of the redrawn map. That means almost one-quarter of potential Democratic gains due to redistricting alone.

Republicans in Majority Democratic Districts

The presidential-year electorate is far more diverse and far younger than in mid-term elections. Almost 32 million more voters cast major-party House ballots in 2012 than they did in 2014. Republicans must defend eight seats they gained in 2014 that are at least 50 percent Democratic, and where lower turnout led to the seat changing hands. The freshmen in many of these districts that lean Democratic in presidential years could be unseated this year.

Contests Without an Incumbent

Open seats in any election provide critical pick-up opportunities, and this year is no different. In districts without an established incumbent, the presidential contest may determine which party wins these open seats. Democrats are at an advantage here as well, with only two currently-held open seats in jeopardy (Arizona’s 1st and Florida’s 18th).

The expanded Republican majority leaves them vulnerable in marginal districts vacated by Republican incumbents. This group can divided into three segments:

• Suburban districts where the presidential race helps the Democratic candidate
• Lower-middle class, largely white districts where Trump is not a liability, and the electorate is divided almost equally between Republicans and Democrats
• Republican leaning districts likely to be retained by the Republicans barring a Trump implosion.

Vulnerable Republican Incumbents

Next is a critical group of suburban Republican incumbents who may win reelection despite the negative fallout from the Trump campaign. They represent districts that are largely upscale demographically, but are still subject to defeat given a Trump implosion at the top of the ticket.

Emerging Hispanic Constituencies

Minority turnout will be a big indicator of Democratic success in this election. A large Hispanic turnout fueled by the Trump campaign might upend Republican incumbents in these three California districts.

All of this means that Democrats will gain seats—probably 10 to 20. Splitting the difference, a gain of 15 seats would be a positive outcome given the paucity of marginal districts as a result of redistricting vagaries.