Rep. Rodney FrelinghuysenRodney Procter FrelinghuysenGOP revolts multiply against retiring Ryan Budget chairman Womack eyes appropriations switch Trump’s budget chief talks spending clawback with key chairman MORE (R-N.J.), the chairman of the powerful House Appropriations Committee who defied his party to vote against President TrumpDonald John TrumpPompeo to outline post-deal strategy on Iran Trump asking aides whether he should proceed with North Korea summit: report Stopping Robert Mueller to protect us all 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 ClintonStopping Robert Mueller to protect us all Hillary Clinton hits Trump, pulls out Russian hat during Yale speech Giuliani: Mueller plans to wrap up Trump obstruction probe by Sept. 1 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 IssaDems step up efforts to avoid California primary shutouts House Dems boost spending in key California races NC Republican pressed on Trump in primary showdown MORE (Calif.) and Ed RoyceEdward (Ed) Randall RoyceDem peace deal reached in crucial House district Dems step up efforts to avoid California primary shutouts House Dems boost spending in key California races 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 WalkerFood stamp revamp sparks GOP fight over farm bill House chaplain is a champion of true Catholicism — Paul Ryan is not Dems put squeeze on Ryan over chaplain controversy 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 StiversGood economic vibes fail to make GOP tax law popular Conservatives warn leadership to stay out of Ohio GOP primary Republicans hold on to Arizona House seat 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-LehtinenOvernight Defense: Over 500 amendments proposed for defense bill | Measures address transgender troops, Yemen war | Trump taps acting VA chief as permanent secretary Defense bill amendment would protect open transgender military service Tensions on immigration erupt in the House GOP MORE (Fla.), Dave ReichertDavid (Dave) George ReichertGOP lawmakers back discharge petition to force immigration votes Major GOP super PAC expands field offices to 31 districts With bills on the table, Congress must heed the call to fix our national parks MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyOvernight Defense: Over 500 amendments proposed for defense bill | Measures address transgender troops, Yemen war | Trump taps acting VA chief as permanent secretary Arizona Senate GOP candidate paid Gorka ,240 to appear at event Trump’s White House shakes off boogeyman Roy Moore with West Virginia win 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 LoBiondoCongress — when considering women’s health, don’t forget about lung cancer GOP revolts multiply against retiring Ryan GOP split on immigration is a crisis for Ryan’s team MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentBudget chairman Womack eyes appropriations switch Primary win gives resurgent left a new shot of adrenaline Who is the farthest left of them all? MORE (Pa.) and Dave TrottDavid Alan TrottGOP split on immigration is a crisis for Ryan’s team Pelosi: GOP discharge-petition holdouts helping Ryan ‘save face’ Two more Republicans back immigration discharge petition 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.