Rep. Rodney FrelinghuysenRodney Procter FrelinghuysenBottom line Republican lobbying firms riding high despite uncertainty of 2020 race Ex-Rep. Frelinghuysen joins law and lobby firm MORE (R-N.J.), the chairman of the powerful House Appropriations Committee who defied his party to vote against President TrumpDonald TrumpDOJ asks Supreme Court to revive Boston Marathon bomber death sentence, in break with Biden vow Biden looking to build momentum for Putin meeting DOJ tells media execs that reporters were not targets of investigations 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 ClintonNSA leaker Reality Winner released from federal prison Monica Lewinsky signs production deal with 20th TV Police investigating death of TV anchor who uncovered Clinton tarmac meeting as suicide 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 Issa'If this thing qualifies, I'm toast': An oral history of the Gray Davis recall in California House Republicans urge opposition to vaccine patent waiver Republicans need to stop Joe Biden's progressive assault on America MORE (Calif.) and Ed RoyceEdward (Ed) Randall RoyceCalifornia was key factor in House GOP's 2020 success Top donor allegedly sold access to key politicians for millions in foreign cash: report Here are the 17 GOP women newly elected to the House this year 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 WalkerTrump endorses Rep. Ted Budd for Senate in North Carolina 9 Senate seats most likely to flip in 2022 The Hill's Morning Report - Biden to country: 'Turning peril into possibility' 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 StiversNew Mexico Democrat Stansbury sworn into Haaland's old seat Retired GOP representative: I won't miss the circus, but I might miss some of the clowns The Hill's Morning Report - Census winners and losers; House GOP huddles 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-LehtinenBottom line Bottom line Democrats elect Meeks as first Black Foreign Affairs chairman MORE (Fla.), Dave ReichertDavid (Dave) George ReichertRep. Kim Schrier defends Washington House seat from GOP challenger Washington Rep. Kim Schrier wins primary Mail ballot surge places Postal Service under spotlight MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyMcGuire unveils Arizona Senate campaign On The Trail: Arizona is microcosm of battle for the GOP Gabby Giffords and Mark Kelly welcome first grandchild 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 LoBiondoVan Drew-Kennedy race in NJ goes down to the wire Van Drew wins GOP primary in New Jersey Amy Kennedy wins NJ primary to face GOP's Van Drew MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentThe Memo: What now for anti-Trump Republicans? Influential Republicans threaten to form new party Loyalty trumps policy in Stefanik's rise, Cheney's fall 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.