Rep. Rodney FrelinghuysenRodney Procter FrelinghuysenFemale vets could lead the change in 2020 Dem Mikie Sherrill wins open-seat NJ House race GOP House candidate receives letter threatening his children MORE (R-N.J.), the chairman of the powerful House Appropriations Committee who defied his party to vote against President TrumpDonald John TrumpMeet the lawyer Democrats call when it's recount time Avenatti denies domestic violence allegations: 'I have never struck a woman' Trump names handbag designer as ambassador to South Africa MORE's tax cuts, announced Monday that he will not seek reelection.

Frelinghuysen, 71, was a top Democratic target given the tilt of his district, which Trump won over Hillary ClintonHillary Diane Rodham ClintonMeet the lawyer Democrats call when it's recount time Overnight Defense — Presented by Raytheon — Mattis defends border deployment during visit to troops | Bolton aide exits WH after clash with first lady | House blocks Yemen war resolution | Report warns of erosion in US military superiority Exit polls show more women breaking with Republicans MORE in 2016 by just a percentage point.

Trump's low approval ratings only raised Democratic hopes that they could defeat Frelinghuysen if he sought a 13th term — especially after passage of a tax-cut bill that puts a $10,000 ceiling on the exemption for local and state taxes and property taxes, which was expected to hit the Republican's district hard.

Frelinghuysen had touted his bipartisan credentials, but his decision to retire suggests he faced a difficult path at best to win reelection.

“I have worked in a bipartisan manner, not just in times of crisis but always, because I believe it best serves my constituents, my state and our country," Frelinghuysen said in a statement.

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"I have sincerely endeavored to earn that trust every day and I thank my constituents and my home state of New Jersey for the honor to serve and I will continue to do so to the best of my abilities through the end of my term," he said.

He follows endangered GOP lawmakers such as Reps. Darrell IssaDarrell Edward IssaDem Mike Levin wins Issa's open House seat in southern California Handful of races remain undecided a day after midterms Trump supporters celebrate and party amid election returns MORE (Calif.) and Ed RoyceEdward (Ed) Randall RoyceHistoric class storms Capitol Election Countdown: Florida braces for volatile recount | Counties race to finish machine recount | Trump ramps up attacks | Abrams files new lawsuit in Georgia | 2020 to be new headache for Schumer | Why California counts its ballots so slowly Republicans jockey for top GOP spot on House Foreign Affairs Committee MORE (Calif.) in deciding to retire. Frelinghuysen is only in his first term as House Appropriations Committee chairman, a top post that lawmakers traditionally serve in for years.

Democrats need a net gain of 24 seats in this fall's midterms to win back control of the House. 

Frelinghuysen has found himself at odds with the House GOP conference repeatedly over the past year.

After initially opposing a GOP ObamaCare repeal-and-replace bill, Frelinghuysen voted for his party's ObamaCare legislation in May — and then came under criticism from activists in his district.

He was one of 12 House Republicans to vote against the GOP’s tax overhaul in December, and warned at the time that it would lead to tax increases for his constituents and “damage our state’s housing market and business environment.”

House GOP leaders considered removing Frelinghuysen as chairman of the Appropriations Committee in retaliation for bucking the party line on one of their biggest legislative priorities.

Frelinghuysen’s vote rubbed his colleagues the wrong way because committee chairmen are generally expected to support the leadership.

Rep. Mark WalkerBradley (Mark) Mark WalkerMcCarthy defeats Jordan for minority leader in 159-to-43 vote Scott to recuse himself from certifying Florida election results House GOP returns to Washington after sobering midterm losses MORE (R-N.C.), chairman of the conservative Republican Study Committee, told The Hill at the time that he had “real problems” with Frelinghuysen voting against the tax bill.

“This is a committee chairman who’s going to be pitching some kind of spending thing, and if you can’t get on board and support one of the promises we’ve made to the American people, I have real problems with that,” Walker said.

Frelinghuysen is now the ninth House committee chairman to opt against seeking reelection this year. But unlike most of the other committee chairmen who are retiring, Frelinghuysen would not have had to relinquish his gavel next year due to the GOP’s term-limit rules that only allow committee chairmen to serve for three consecutive terms.

His retirement could make it easier for Democrats to take advantage of an open seat this election cycle instead of attempting to topple a 12-term incumbent.

National Democrats are rallying around Mikie Sherrill, who had already launched a campaign against Frelinghuysen.

“Rep. Frelinghuysen’s retirement opens up a very competitive seat that is moving quickly toward Democrats. With veteran and former federal prosecutor Mikie Sherrill’s strong candidacy, and the abysmal approval ratings of Speaker [Paul] Ryan’s [R-Wis.] Republican Congress, Democrats are confident that this seat will turn blue [in] November,” Democratic Congressional Campaign Committee spokesman Evan Lukaske said.

Republicans, however, expressed confidence that they could keep a longtime GOP seat in their column.

"This district has been held by a Republican since the 1980s, and we plan to keep it that way in November,” National Republican Congressional Committee Chairman Steve StiversSteven (Steve) Ernst StiversHouse GOP returns to Washington after sobering midterm losses Conservative magazine posts recording of King using derogatory language against immigrants Steve King accuses Weekly Standard of trying to ‘advance a Leftist agenda’ MORE (R-Ohio) said in a statement.

Frelinghuysen’s district had already been rated a “toss up” by the Cook Political Report, a nonpartisan election prognosticator.

His retirement is yet another boon for Democrats, who are seeking to take advantage of a growing number of open seats in districts already considered competitive.

Six House Republicans representing districts won by Clinton in 2016 won’t be seeking reelection, more than half of whom announced their plans within the last few weeks. They include Royce, Issa and Reps. Ileana Ros-LehtinenIleana Carmen Ros-LehtinenHouse GOP returns to Washington after sobering midterm losses Dem wins leave behind a more conservative GOP conference How Republicans who voted against ObamaCare repeal fared in midterms MORE (Fla.), Dave ReichertDavid (Dave) George ReichertHow Republicans who voted against ObamaCare repeal fared in midterms Election Countdown: Recount prospects grow in Florida | Abrams team to sue over absentee ballots | Dem wins pivotal Georgia House seat | A look at the uncalled races | Corporations spend big to beat ballot measures Democrat Kim Schrier flips Washington House district MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyHouse GOP returns to Washington after sobering midterm losses Overnight Defense — Presented by Raytheon — First lady's office pushes for ouster of national security aide | Trump taps retired general as ambassador to Saudis | Mattis to visit border troops | Record number of female veterans to serve in Congress Election Countdown: Lawsuits fly in Florida recount fight | Nelson pushes to extend deadline | Judge says Georgia county violated Civil Rights Act | Biden, Sanders lead 2020 Dem field in poll | Bloomberg to decide on 2020 by February MORE (R-Ariz.), meanwhile, is running for the Senate instead of seeking reelection in her competitive Clinton-carried district.

Other open swing seats eyed by Democrats include those currently held by retiring Republican Reps. Frank LoBiondoFrank Alo LoBiondoHow Republicans who voted against ObamaCare repeal fared in midterms Democrat Jeff Van Drew wins in New Jersey district House GOP group cuts financial support for Coffman, Bishop MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentDem wins leave behind a more conservative GOP conference How Republicans who voted against ObamaCare repeal fared in midterms Democrat Susan Wild wins House race in Pennsylvania MORE (Pa.) and Dave TrottDavid Alan TrottDemocrats flip Michigan seat in race between two political newcomers Election Countdown: Cruz, O'Rourke fight at pivotal point | Ryan hitting the trail for vulnerable Republicans | Poll shows Biden leading Dem 2020 field | Arizona Senate debate tonight House battlefield expands as ad wars hit new peak MORE (Mich.).

In another sign of a potentially difficult year for the party, House Republicans will have to deal with far more open seats this election cycle than Democrats.

House Republicans will have to defend at least 35 open seats this year due to retirements, resignations and lawmakers running for another office. Democrats, meanwhile, have less than half as many open seats as they seek to expand their electoral map. 

Updated at 3:20 p.m.