Rep. Rodney FrelinghuysenRodney Procter FrelinghuysenThe 31 Trump districts that will determine the next House majority Top House GOP appropriations staffer moves to lobbying shop Individuals with significant disabilities need hope and action MORE (R-N.J.), the chairman of the powerful House Appropriations Committee who defied his party to vote against President TrumpDonald John TrumpKaine: Obama called Trump a 'fascist' during 2016 campaign Kaine: GOP senators should 'at least' treat Trump trial with seriousness of traffic court Louise Linton, wife of Mnuchin, deletes Instagram post in support of Greta Thunberg 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 ClintonKaine: Obama called Trump a 'fascist' during 2016 campaign Clinton says Zuckerberg has 'authoritarian' views on misinformation Des Moines Register endorses Elizabeth Warren as Democratic presidential nominee 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 IssaDuncan Hunter to plead guilty to campaign finance violations Why the GOP march of mad hatters poses a threat to our Democracy Elijah Cummings, native son of Baltimore, gets emotional send-off from Democratic luminaries MORE (Calif.) and Ed RoyceEdward (Ed) Randall RoyceThe most expensive congressional races of the last decade Mystery surrounds elusive sanctions on Russia Hillicon Valley: Lawmakers struggle to understand Facebook's Libra project | EU hits Amazon with antitrust probe | New cybersecurity concerns over census | Robocall, election security bills head to House floor | Privacy questions over FaceApp 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 WalkerRepublicans, Democrats offer support after John Lewis cancer diagnosis House GOP vows to use impeachment to cut into Democratic majority A solemn impeachment day on Capitol Hill 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 StiversKoch campaign touts bipartisan group behind ag labor immigration bill Waters clashes with Trump officials over 'disastrous' housing plans Financial sector's work on SAFE Banking Act shows together, everyone achieves more 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-LehtinenVeronica Escobar to give Spanish-language response to Trump State of the Union address Ex-Rep. Duffy to join lobbying firm BGR Former GOP Rep. Walters joins energy company MORE (Fla.), Dave ReichertDavid (Dave) George ReichertBottom Line The most expensive congressional races of the last decade Lymphedema Treatment Act would provide a commonsense solution to a fixable problem MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyDemocrats feel political momentum swinging to them on impeachment Senate Republicans confident they'll win fight on witnesses How Citizens United altered America's political landscape 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 LoBiondo Democratic challenger on Van Drew's party switch: 'He betrayed our community' Trump announces Van Drew will become a Republican in Oval Office meeting Van Drew, set to switch parties, will vote as a Democrat on impeachment MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentThe biggest political upsets of the decade Ex-GOP lawmaker: Former colleagues privately say they're 'disgusted and exhausted' by Trump Overnight Health Care — Presented by Better Medicare Alliance — Federal judge blocks Trump from detaining migrant children indefinitely | Health officials tie vaping-related illnesses to 'Dank Vapes' brand | Trump to deliver health care speech in Florida MORE (Pa.) and Dave TrottDavid Alan TrottFormer GOP Michigan congressman says Trump is unfit for office Pro-Trump Republican immigrant to challenge Dem lawmaker who flipped Michigan seat Meet the lawmakers putting politics aside to save our climate 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.