Rep. Rodney FrelinghuysenRodney Procter FrelinghuysenDems eyeing smaller magic number for House majority Puerto Rico mayor: Territory's profile has grown since hurricanes House panel advances homeland security bill with billion in border wall funding MORE (R-N.J.), the chairman of the powerful House Appropriations Committee who defied his party to vote against President TrumpDonald John TrumpTrump threatens ex-intel official's clearance, citing comments on CNN Protesters topple Confederate monument on UNC campus Man wanted for threatening to shoot Trump spotted in Maryland 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 ClintonClinton to headline trio of DNC fundraisers: report Allegations of ‘Trump TV’ distract from real issues at Broadcasting Board of Governors Chelsea Clinton: Politics a 'definite maybe' in the future 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 IssaTrump says DOJ official should be fired over role in Russia probe Dems eyeing smaller magic number for House majority Dems make big play for House in California MORE (Calif.) and Ed RoyceEdward (Ed) Randall RoyceAllegations of ‘Trump TV’ distract from real issues at Broadcasting Board of Governors Steyer group launching 0,000 digital ad campaign targeting millennials It’s possible to protect national security without jeopardizing the economy 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 WalkerThree scenarios for how leadership races could play out in the House Florida university to get early voting site after judge strikes down ban Student voter suppression is an affront to the memory of Andrew Goodman 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 StiversTop GOP lawmaker: Tax cuts will lower projected deficit Trump sends, deletes tweet urging Ohio voters to support candidate not in special election GOP rep: ‘Things are moving our way’ ahead of midterms 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-LehtinenMiami Herald endorses House candidate who claims aliens took her aboard a spaceship House GOP starts summer break on a note of friction Rosenstein impeachment push divides House GOP leadership MORE (Fla.), Dave ReichertDavid (Dave) George ReichertElection Countdown: GOP worries House majority endangered by top of ticket | Dems make history in Tuesday's primaries | Parties fight for Puerto Rican vote in Florida | GOP lawmakers plan 'Freedom Tour' Democrat Kim Schrier advances in Washington primary Overnight Energy: Koch backs bill opposing carbon taxes | Lawmakers look to Interior budget to block offshore drilling | EPA defends FOIA process MORE (Wash.) and Patrick Meehan (Pa.).

Rep. Martha McSallyMartha Elizabeth McSallyTrump stays out of Arizona's ugly and costly GOP fight States are stepping up to end animal testing in cosmetics while federal legislation stalls GOP Senate candidate truncates Trump tweet in campaign mailer 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 LoBiondoDems eyeing smaller magic number for House majority GOP campaign arm withdraws support from NJ House candidate who made racist statements GOP runs into Trump tax law in New Jersey MORE (N.J.), Charlie DentCharles (Charlie) Wieder DentGOP House candidate placed on leave from longtime position after sexual misconduct allegation Election handicapper moves GOP leader's race to 'toss-up' The Hill's 12:30 Report MORE (Pa.) and Dave TrottDavid Alan TrottRecord numbers of women nominated for governor, Congress Key primaries in August will help shape midterms GOP doubles female recruits for congressional races 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.