The magic number of seats Democrats need to hit to win back the House majority is 23. But in reality, it’s almost certainly a much smaller number.

Several GOP-held seats are seemingly already in the bag, meaning Democrats likely need to take a smaller number of competitive seats — perhaps as few as 15 — to return to the majority for the first time since 2010.

It’s all making Republicans nervous — even before Tuesday’s special election in Ohio, where a Republican candidate appears to have just scraped by in a district that has been in GOP hands since 1983 and that President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE won two years ago by 11 points.

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“I don’t think there is necessarily a blue wave, but what concerns me is suburban districts in New Jersey, Philadelphia, New York, Illinois and California — those are not good for us,” said one House Republican from a blue state.

“Educated women, young people independents — they are energized. I clearly think [Democrats] have enough to win over 24 seats.”

Democrats, for their part, are taking nothing for granted, saying it’s well too soon to divert resources from any districts.

“The biggest strategic challenge [Democrats] have will come in September and October when they’ve got to make a decision whether some races are now in the safe column and they can divert resources to lean-Republican races,” said former Rep. Steve Israel Steven (Steve) J. IsraelThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' The Hill's Morning Report - Presented by Facebook - Trump, Biden intensify battleground focus as 2020 race tightens Biden allies express confidence as convention begins MORE (D-N.Y.).

“It’s way too early to make those decisions,” added Israel, who led the Democratic Congressional Campaign Committee (DCCC) from 2011 to 2014.

According to campaign prognosticators, the GOP seats most likely to flip are held by three retiring Republicans.

The first, a New Jersey seat being vacated by longtime Rep. Frank LoBiondo Frank Alo LoBiondoVan Drew wins GOP primary in New Jersey Amy Kennedy wins NJ primary to face GOP's Van Drew Stimulus price tag of .2T falls way short, some experts say MORE, has a GOP nominee seen as exceptionally weak. The House GOP’s campaign arm rescinded its support after reports that the nominee had shared racist articles on social media. The Cook Political Report rates it as "likely Democratic."

Cook also lists seats being vacated by GOP Reps. Pat Meehan Patrick (Pat) Leo MeehanBottom line Freshman lawmaker jokes about pace of Washington politics Many authors of GOP tax law will not be returning to Congress MORE and Ryan Costello Ryan Anthony CostellloTrump struggles to stay on script, frustrating GOP again Bottom line Former GOP Rep. Costello launches lobbying shop MORE in Pennsylvania as "likely Democratic." Both Republicans saw their districts redrawn in favor of Democrats in a gerrymandering court case this year.

Four more seats are listed by both Cook and Sabato’s Crystal Ball of the University of Virginia, another handicapper, as "leaning Democratic." All were won by Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE in 2016.

The four GOP seats are held by Reps. Barbara Comstock Barbara Jean ComstockLive coverage: House holds third day of public impeachment hearings Gun debate raises stakes in battle for Virginia legislature Progressives face steep odds in ousting incumbent Democrats MORE, who represents a blue-tinged Northern Virginia district outside of D.C.; Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (Ariz.), who’s running for the Senate; and retiring longtime Reps. Ileana Ros Lehtinen (R-Fla.) and Darrell Issa Darrell Edward IssaDCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program Wife of former Rep. Duncan Hunter sentenced to 8 months of home confinement Harris endorses Democrat in tight California House race MORE (R-Calif.).

The three other GOP seats listed by Cook as "leaning Democrat" are in New Jersey, where Rep. Rodney Frelinghuysen Rodney Procter FrelinghuysenBottom line Republican lobbying firms riding high despite uncertainty of 2020 race Ex-Rep. Frelinghuysen joins law and lobby firm MORE is retiring, and in Pennsylvania, where the redistricting decision is adding to the GOP’s problems.

One open seat in Pennsylvania now held by Democrats is rated as "likely Republican" because of the redrawn districts.

The Ohio race, coming on the heels of Democratic Rep. Conor Lamb’s March victory in a deep-red Pennsylvania district, has Democrats believing the battlefield is expanding quickly in their favor.

“The conventional wisdom up until several months ago was that it was fairly easy to find 20 seats that were low-hanging fruit for Democrats, but those final four or five that they needed required a reach,” Israel said. “When you are competitive in Ohio-12, and you’re winning districts like Conor Lamb’s, that suggests that you’re not going to have to reach that high to find those five seats.”

Publicly, Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) and his leadership team are predicting that Trump’s tax cuts and the strong economy will help them stave off a Democratic wave.

Privately, Republicans are more nervous — though they think that whoever has the majority will win it by just a few seats.

“It’s 50-50,” said the GOP lawmaker. “And whoever wins, it’s going to be a very small governing margin.”

History and math are not in the GOP’s favor. The party that controls the White House typically loses dozens of seats during a president’s first midterm election. And President Trump’s approval rating is in the low 40s, a drag on Republicans in districts leaning toward the Democrats, like Comstock’s.

On top of that, a wave of GOP retirements, some unexpected, have put Republicans in the unenviable position of having to defend 42 open seats this cycle, eight of which are in districts that Clinton won in 2016 and more than a dozen others in districts that Trump barely won.

The spate of GOP retirements creates more uncertainty and gives Democrats defending half as many open seats more paths to the majority.

“We have a lot of members who would retain their seats if they had stayed, but instead of having a member who raises their own money and wins their race, you replace that with a newcomer,” Rep. Daniel Webster Daniel Alan WebsterGaetz set to endorse primary opponent of fellow Florida GOP lawmaker Former cop Demings faces progressive pushback in veepstakes Overnight Energy: Biden campaign says he would revoke Keystone XL permit | EPA emails reveal talks between Trump officials, chemical group before 2017 settlement | Tensions emerge on Natural Resources panel over virtual meetings MORE (R-Fla.) told The Hill. “It’s making it very difficult from a financial standpoint. We have to go out and hold on to the seat instead of counting it as an automatic.”

Even well-known, politically powerful GOP incumbents are struggling. More than 50 of them were outraised by Democratic challengers in the second fundraising quarter of 2018 that ended June 30. They include veteran Rep. John Carter John Rice CarterDonna Imam wins Democratic runoff to face Rep. John Carter House panel advances bill banning construction on bases with Confederate names Democrats see victory in Trump culture war MORE (R-Texas) and Rep. John Culberson John Abney Culberson2020 Democratic Party platform endorses Trump's NASA moon program Bottom line Ex-Rep. Frelinghuysen joins law and lobby firm MORE (R-Texas) — both senior appropriators — House Small Business Committee Chairman Steve Chabot Steven (Steve) Joseph ChabotKate Schroder in Ohio among Democratic challengers squelching GOP hopes for the House Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill Lawmakers call for expanded AI role in education, business to remain competitive MORE (R-Ohio) and Rep. Leonard Lance Leonard LanceThomas Kean wins GOP primary to take on Rep. Tom Malinowski Gun debate to shape 2020 races GOP fears Trump backlash in suburbs MORE (R-N.J.).

Still, Democrats face plenty of challenges of their own. They're defending two open Minnesota seats this fall that went overwhelmingly for Trump in 2016; Cook and Sabato have put them in the “toss up” column.

Former Rep. Rahm Emanuel (D), now the Chicago mayor, told Democrats in a briefing earlier this year that the political environment is similar to that of 2006, when he chaired the DCCC and the Democrats took control of the lower chamber.

But Emanuel also warned that the congressional map has shifted sharply in the GOP’s favor since 2006, creating a much steeper climb for Democrats.

“Without the redistricting firewall that they built, Republicans would probably lose 60 seats in this kind of cycle,” Israel said. “As a result of the redistricting firewall that they built, they could lose about half of that.”

Senior Republicans like Chabot said they are taking their races seriously but don’t think they’ll be ousted in November. The GOP chairman believes voters are reaping the rewards from a GOP-controlled government with tax and regulations cuts and an improving economy, and that they’ll stick with Republicans this fall.

“Most of us are doing the things necessary to be able to prevail in this kind of environment,” said Chabot, who represents most of Cincinnati and some suburbs. “If we were asleep at the switch and didn’t see this coming, it could be a real problem. But I think many of us are ready."