House Minority Leader Nancy Pelosi, D-Calif. Tom Williams | CQ-Roll Call Group | Getty Images

With most of 2018's primaries done, top election forecasters agree Democrats have an edge in the fight for a House majority. Still, a lot could happen between now and the Nov. 6 midterm elections to help Republicans hang on to enough seats to keep control of the chamber. The Democratic Party has a clear task in front of it: win at least 23 GOP-held seats and flip the House. History and current trends virtually guarantee Republicans will lose seats in November. The question is whether Democrats will take enough seats to gain control of the chamber, or fall short in their effort. Buoyed by President Donald Trump's relatively poor approval rating, lackluster views on the congressional GOP's policy priorities and historical difficulties for the president's party in midterms, Democrats have about an 8-percentage point edge on national generic ballots asking voters which party they would prefer, according to a FiveThirtyEight average. One of the data site's forecasts, which takes into account polling, fundraising, voting history and analyst ratings at the House district level, gives Democrats a 68.5 percent chance of taking the House. It projects an average gain of 31 seats. "I do think the likeliest outcome is [Democrats] take a majority of some size. ... You'd rather be the Democrats than the Republicans now in the race for the House," said Kyle Kondik, managing editor of Sabato's Crystal Ball, a top nonpartisan election analysis site that assigns ratings to every House race. But Democrats have hardly locked up a House majority with more than two months to go. An improvement in Trump's approval rating or for Republicans in the generic ballot, along with surprise twists at the national or district level, could help the GOP to cling to its House majority, according to Kondik and Leah Askarinam, a reporter and analyst at top nonpartisan analysis site Inside Elections. Democrats have little chance of taking a majority in the Senate, as the party's senators and independents who caucus with them defend 26 seats, including 10 in states Trump won in 2016. Only nine Republican Senate seats are up for grabs this year. In that environment, the fight for House control has become even more important. The stakes are huge for the president and his agenda. A Democratic House could not only block Trump's priorities, but also push for its own goals such as patching up the Affordable Care Act or reversing some GOP tax cuts for corporations. Crucially, Democrats would have the power to start committee investigations or push for impeachment — an issue the party is taking pains to avoid now as it tries not to stir up Republican voter enthusiasm.

Why Democrats are favored to take the House now

Numerous factors have given Democrats an inside track to the House majority. To start, presidents' parties typically get clobbered in non-presidential election years. In some recent examples, Republicans gained a staggering 64 seats in 2010 when President Barack Obama was in office, while Democrats took 32 in 2006 during President George W. Bush's second term. On top of that, Trump's popularity has lagged behind most of his recent predecessors at this point in their first terms. He has about 42 percent approval versus 53 percent disapproval, according to a rolling FiveThirtyEight average of polls. The majority Republican Congress's initiatives also have not landed with voters the way the party hoped. The GOP's biggest accomplishment, its overhaul of the U.S. tax system, has about 37 percent approval versus 43 percent disapproval, according to a RealClearPolitics polling average. In addition, support for the Affordable Care Act, known as Obamacare, only grew during Trump's presidency as Republicans tried to repeal it and replace it with less popular alternatives. It has about 50 percent support now, versus 40 percent opposition, according to a RealClearPolitics polling average. District-level dynamics also tilt in favor of Democrats. The GOP has to defend 25 seats that Democrat Hillary Clinton won in 2016, while Democrats have to protect just 13 seats that Trump carried, per Sabato's Crystal Ball. In addition, retirements, resignations or the primary election defeat of an incumbent have made the environment more challenging for Republicans in swing districts. Sabato's Crystal Ball rates 14 GOP-held seats in which an incumbent is not running as either a toss-up or favoring Democrats. The same can be said for only three Democratic-held seats.

Incumbents generally fare better in elections than newcomers. Add to the retirements the fact that Pennsylvania's Supreme Court threw out a GOP-drawn congressional map in favor of one friendlier to Democrats, and it puts plenty more seats in play. Fundraising — generally a strong indicator of success in House races — bodes well for Democrats, too. The party's candidates have amassed more cash in individual contributions than Republicans in 71 of 101 districts the nonpartisan Cook Political Report considers competitive, according to FiveThirtyEight's Nate Silver. Challengers typically struggle to raise more than current members of Congress, but Republican incumbents are running in about two-thirds of those districts, he adds. That helps Democrats not only because pro-Democratic outside groups have to spend less money, but also because pro-GOP independent organizations have to shell out more to keep up, said Inside Elections' Askarinam. "When Democratic candidates are able to carry their own in fundraising that means outside groups don't have to carry as much weight in bailing them out. They're able to target more places. The more places you target, the more places you're able to win," she said. Republicans have had their own fundraising triumphs this year. The Republican National Committee said this week that it raised $14.2 million in July, a record for a non-presidential election year. The party said it transferred $4 million each to the National Republican Senatorial Committee and the National Republican Congressional Committee in July. The organizations are dedicated to electing GOP candidates to the Senate and House, respectively. Outside groups such as the NRCC or Congressional Leadership Fund super PAC can swoop in to boost GOP candidates who face a fundraising disadvantage, as seen recently in Republican Troy Balderson's narrow victory in a special election for the red Ohio 12th District. Still, Askarinam notes that outside groups will have a difficult time bailing out GOP candidates in dozens of races across the country.

What could go wrong for Democrats

While every major nonpartisan forecaster believes Democrats are at least slight favorites to take a House majority, they all note that the party's control of the House is far from a sure thing. Republicans have one major factor going for them: a strong U.S. economy. In July, job growth continued at a steady pace, while the unemployment rate dipped slightly to 3.9 percent, around its lowest in 50 years. U.S. gross domestic product also grew by 4.1 percent in the second quarter. While a solid economy may not save Republicans, it will at least make this year's environment a little easier for them, Kondik and Askarinam said. "Right now, that's something Republicans can rely on. ... They may not be campaigning on that as their main issue, but it's not something they have to defend," Askarinam said. Still, Kondik said that "this election isn't about the economy right now, it's about the president." Trump's approval rating is poor, and the legal scrutiny he faces after his ex-personal lawyer Michael Cohen implicated him in a crime will do little to help. Trump has denied any wrongdoing.