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 TrumpSenate gears up for battle over witnesses in impeachment trial Vulnerable Democrats tout legislative wins, not impeachment Trump appears to set personal record for tweets in a day 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 ClintonBiden hires Clinton, O'Rourke alum as campaign's digital director Trump neck and neck with top 2020 Democrats in Wisconsin: poll Clinton tweets impeachment website, encourages voters to 'see the evidence for themselves' 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 RoyceMystery 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 House panel advances bill to protect elections from foreign interference 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 WalkerThe Hill's Campaign Report: Democrats worry about diversity on next debate stage North Carolina congressman says he won't seek reelection after redistricting NC rep explores Tillis primary challenge 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-LehtinenEx-Rep. Duffy to join lobbying firm BGR Former GOP Rep. Walters joins energy company Republican Salazar seeks rematch with Shalala in key Miami House district MORE (Fla.), Dave ReichertDavid (Dave) George ReichertLymphedema Treatment Act would provide a commonsense solution to a fixable problem Yoder, Messer land on K Street Ex-GOP lawmaker from Washington joins lobbying firm MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyWhite House makes push for paid family leave and child care reform The Hill's Morning Report - Sponsored by AdvaMed - House panel expected to approve impeachment articles Thursday Democrats spend big to put Senate in play 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 LoBiondoFormer GOP Rep. Costello launches lobbying shop Republicans plot comeback in New Jersey K Street giants scoop up coveted ex-lawmakers MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentEx-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 The Hill's Morning Report — Mueller testimony gives Trump a boost as Dems ponder next steps MORE (Pa.) and Dave TrottDavid Alan TrottPro-Trump Republican immigrant to challenge Dem lawmaker who flipped Michigan seat Meet the lawmakers putting politics aside to save our climate Michigan New Members 2019 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.