Rep. Rodney FrelinghuysenRodney Procter FrelinghuysenExiting lawmakers put in calls to K Street Ex-New York Jets lineman mulling run for House SEC paperless mandate a bad deal for rural, elderly investors MORE (R-N.J.), the chairman of the powerful House Appropriations Committee who defied his party to vote against President TrumpDonald John TrumpAccuser says Trump should be afraid of the truth Woman behind pro-Trump Facebook page denies being influenced by Russians Shulkin says he has White House approval to root out 'subversion' at VA 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 ClintonWoman behind pro-Trump Facebook page denies being influenced by Russians Trump: CNN, MSNBC 'got scammed' into covering Russian-organized rally Pennsylvania Democrats set to win big with new district map 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 IssaCalifornia Hispanics, even Republicans, are primed to make history House rejects effort to condemn lawmaker for demanding 'Dreamer' arrests Hispanic Dems seek vote to condemn GOP lawmaker for demanding arrests of 'Dreamers' MORE (Calif.) and Ed RoyceEdward (Ed) Randall RoyceRetiring GOP rep: ‘I think we should look at maybe the length of our chairmanships’ Outgoing GOP rep: Republican Party 'heading into trouble' in election Sunday shows preview: Russian charges, Florida shooting dominate coverage 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 WalkerFlorida shooting reopens CDC gun research debate Right revolts on budget deal Judge overturns ex-felon voting rights process in Florida 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 Republicans add 5 members to incumbent protection program Polls swing toward GOP, easing fears of midterm disaster GOP turns Pelosi's words into weapon for tax law 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-LehtinenOutgoing GOP rep: Republican Party 'heading into trouble' in election Sunday shows preview: Russian charges, Florida shooting dominate coverage House rejects effort to condemn lawmaker for demanding 'Dreamer' arrests MORE (Fla.), Dave ReichertDavid (Dave) George ReichertLawmaker interest in NAFTA intensifies amid Trump moves Lawmakers call on Trump to preserve NAFTA Dems dominate GOP in cash race for key seats MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyPacific Command chief: 'Fair' to criticize Olympic coverage of North Korea Overnight Finance: Shutdown looms | Paul holds up Senate vote | House GOP scrambles for budget votes | What's in the deal | Dow falls 1,000 points for second time this week | Conservatives threaten Fed nominee | Trump announces IRS pick House GOP leaders scramble for budget votes 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 LoBiondoHouse GOP Appropriations chairman calls it quits Democrats have open door amid wave of Republican retirements GOP angst over midterms grows MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentPennsylvania Democrats set to win big with new district map Outgoing GOP rep: Republican Party 'heading into trouble' in election Sunday shows preview: Russian charges, Florida shooting dominate coverage MORE (Pa.) and Dave TrottDavid Alan TrottHouse GOP Appropriations chairman calls it quits Democrats have open door amid wave of Republican retirements GOP angst over midterms grows 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.