Democratic candidate LuAnn Bennett, left, and Rep. Barbara Comstock (R-Va.) greet one another after their first debate in the race for Virginia’s 10th Congressional District in Lansdowne, Va., on Oct. 6. Democrats must unseat Comstock to have any hope of reclaiming the House majority. (Jahi Chikwendiu/The Washington Post)

Hillary Clinton’s persistent advantage over Donald Trump in recent presidential polls has Democrats eyeing big gains in Congress. And while the Senate majority is clearly in sight, the House majority is a much more elusive goal.

Reaching it will require Democratic candidates to triumph in a variety of districts, taking advantage not only of Trump’s weaknesses but also, in many cases, more localized political climates. Even then, Democrats face a tall order in reclaiming a majority, with their best prospects lying largely in suburban areas that are the most expensive places to wage effective campaigns.

Republicans currently hold a 246-to-186 majority; three seats — one reliably Republican, two reliably Democratic — are vacant. That means Democrats need to flip 30 seats, and party strategists have been careful in discussing that prospect. House leaders, including Minority Leader Nancy Pelosi (D-Calif.) have suggested a 20-seat gain would be a good outcome.

Flipping the House would require the confluence of factors — robust Democratic turnout, on a par with the levels seen in 2008 and 2012 for President Obama; a significant depression in Republican turnout to near-midterm levels as voters unenthused by Trump simply decide not to cast ballots; and, to a lesser extent, Trump voters withholding their support from the handful of GOP candidates who have broken with the nominee.

Mark S. Mellman, a Democratic pollster who is consulting on about a dozen House races, said a 10-point Clinton win could put the majority into play. “It’s not a certainty, but it’s certainly a possibility at that stage,” he said.

Republicans, meanwhile, have counseled incumbents to tout their constituent work while campaigning against the persistent unpopularity of both top presidential candidates. “If we had to readjust every time something happened at the top of the ticket, we would be in disarray, so we have been focused on keeping it local,” said Katie Martin, a spokeswoman for the National Republican Congressional Committee.

But GOP members are increasingly playing defense in their advertising, and top leaders sounded alarms last week after the Democratic Congressional Campaign Committee announced a $21 million September fundraising haul. House Majority Leader Kevin McCarthy (R-Calif.) pleaded with his GOP colleagues on a conference call last week for more campaign cash from safer incumbents, saying that Trump’s sagging unpopularity was beginning to hurt vulnerable members.

Still, it’s not as simple as waiting for a Clinton blowout for Democrats. They still must have competent candidates on the ballot, campaigns hand-tailored to individual districts and enough funding to get their message out widely. Here are three key races that represent the different environments Democrats must navigate.

Nevada’s 4th District: The gimme

In 2014, GOP Rep. Cresent Hardy won this district, anchored in Las Vegas’s northern suburbs, when Democrats stayed home. This time, Democrats are confident that their candidate, state lawmaker Ruben Kihuen, will resonate in a district that is at least 55 percent Asian, black and Hispanic. Kihuen was born in Mexico and immigrated with his parents to the United States in the 1980s — a life story that Democrats think will resonate with voters.

“This is a working-class district. This is a majority-minority district. If you look at the people in this district, they’re unemployed, they need jobs, they need their wages increased,” Kihuen said in a recent interview.

The National Republican Congressional Committee has spent at least $4.2 million in support of Hardy, with another $1.4 million reserved in the Las Vegas media market for the final two weeks. A new NRCC spot slams Kihuen for his past association with a controversial lobbying firm.

Kihuen has made Trump a key part of his campaign, and national Democratic groups are spending at least $1.6 million in the final weeks to drive that message — even after Hardy withdrew his support for the presidential candidate after the release of the “Access Hollywood” video. A new ad from the House Majority PAC notes how Hardy stood by Trump through all of his previous controversies.

The only recent public polling in the race comes from the Democratic Congressional Campaign Committee, which said last week that Kihuen held a two-point lead in a district where Clinton is leading Trump by seven points. Kihuen should also benefit from a strong turnout effort, making this among the lowest-hanging fruit for House Democrats. If they can’t win here, they will have a hard time picking up even 15 seats.

Virginia’s 10th District: The suburban “pivot”

Democrats’ long-term plan for reclaiming the House majority has anticipated a “demographic pivot” where well-educated, wealthy suburban dwellers who tended to vote Republican become increasingly Democratic over time. Trump, party strategists argue, has accelerated that pivot to the point that districts thought to be out of reach until 2018 or beyond are now in play.

There’s no better proving ground for that theory than the outer suburbs of Washington, where Republicans are in peril of losing a district they have held since 1980. Incumbent Rep. Barbara Comstock is facing a challenge from LuAnn Bennett, a real estate developer and first-time candidate, and they are on track to spend a combined $14 million in the pricey D.C. media market.

National Republican groups, including the NRCC and the Congressional Leadership Fund super PAC, have already spent $4.1 million painting Bennett as a tax-hiking liberal with a sketchy business record, with another $3.3 million in TV ads reserved for the final weeks.

Democratic groups, meanwhile, have countered with roughly $3 million in TV and radio ads — including spots highlighting Comstock’s positions against abortion rights and same-sex marriage, a message aimed at the suburban voters now seen as a keystone of the Democratic coalition. Another $2.1 million is coming before Election Day.

Bennett’s latest ad declares that “Comstock and Donald Trump are the same.”

Polling in the race has been sparse, but it has been among the most expensive races for both parties. But the ads don’t tell the whole story: The two have sparred about local issues, as well — including who would be better suited to handle transportation issues. At a Northern Virginia Chamber of Commerce debate last week, Comstock — now the only Republican representing the D.C. suburbs — emphasized the importance of having a member of the House majority in the regional delegation.

“That’s kind of a big assumption at this point,” Bennett countered.

Kansas’s 3rd District: The curveball

In a few districts, it’s not just about Trump, it turns out. In a late push to oust incumbent GOP Rep. Kevin Yoder, Democrats are seizing on the unpopularity of Kansas’s Republican governor, Sam Brownback.

Yoder’s opponent, financial consultant and political neophyte Jay Sidie, is arguing to his suburban Kansas City district that Yoder’s trickle-down tax policies are the same as those advocated by Brownback, which have resulted in major cutbacks to state programs.

Sidie is now benefiting from a $1 million DCCC ad campaign that includes a spot tying Yoder to school funding cuts he advocated as a state legislator. Sidie recently issued a poll showing him four points behind in a district where Clinton is well ahead of Trump.

Yoder and Republican allies are set to spend a combined $1.8 million before Election Day. His campaign manager, Cate Duerst, said Sidie is “a disaster of a candidate” with a questionable business record and that Democrats are spinning “relentless lies” about Yoder. “It’s a political ploy that shows just how much contempt and little respect Washington Democrats have for voters in Kansas, and it won’t succeed,” she said. But Yoder has been forced to distance himself from Brownback — he left the Kansas legislature before Brownback became governor — and by attacking Sidie for missing votes on local education funding.

Sidie said in a recent interview that Yoder’s attacks have only served to make him better known to voters yearning for an alternative to Republican mismanagement, and he stands by his attempts to link Yoder to Brownback and Trump.

“Whenever Brownback gives a speech, Yoder is right behind him clapping, and the same thing with Trump,” he said. “I think the people here just aren’t going to put up with that.”

In a sign of unease, the Congressional Leadership Fund last week said it would pump $700,000 into Yoder’s race. It could be money well spent to save the GOP House majority: A Democratic majority would almost certainly need to include Sidie.

This item has been updated.