Longtime Sen. Carl LevinCarl LevinPresident Trump, listen to candidate Trump and keep Volcker Rule Republicans can learn from John McCain’s heroism Trump and GOP wise to keep tax reform and infrastructure separate MORE (D-Mich.) won't seek another Senate term, leaving Democrats to defend a fourth open seat in 2014.

"I have decided not to run for reelection in 2014," Levin said in a statement Thursday afternoon. "This decision was extremely difficult because I love representing the people of Michigan in the U.S. Senate and fighting for the things that I believe are important to them."

Levin is the fourth Democrat to announce his retirement ahead of the 2014 midterms, following Sens. Jay RockefellerJay RockefellerOvernight Tech: Trump nominates Dem to FCC | Facebook pulls suspected baseball gunman's pages | Uber board member resigns after sexist comment Trump nominates former FCC Dem for another term Obama to preserve torture report in presidential papers MORE (D-W.Va.), Frank Lautenberg (D-N.J.) and Tom HarkinTom HarkinDistance education: Tumultuous today and yesterday Grassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream MORE (D-Iowa).

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Levin, a six-term senator and the powerful chairman of the Senate Armed Services Committee, has long been viewed as a possible retirement prospect. He'll be 80 years old by the end of his term, and had raised little money for a possible reelection.

His decision will have major ramifications both in Congress and on the campaign trail. Levin has been a towering figure on foreign affairs, chairing the Armed Services committee from 2001-2003 and again since 2007. Sen. Jack ReedJohn (Jack) Raymond ReedOvernight Defense: Senate passes 0B defense bill | 3,000 US troops heading to Afghanistan | Two more Navy officials fired over ship collisions Senate passes 0B defense bill This week: Senate wrapping up defense bill after amendment fight MORE (D-R.I.) is next in line to head the committee.

Levin said he'd focus his remaining time in Congress on "dealing with fiscal pressures on our military readiness," pushing for a "rapid transfer of responsibility for Afghan security to the Afghans," ending corporate tax avoidance schemes, helping Michigan's manufacturing rebound and seeking to limit secret money in politics.

"If you’ve ever worn the uniform, worked a shift on an assembly line, or sacrificed to make ends meet, then you’ve had a voice and a vote in Senator Carl Levin," President Obama said in a statement.

"No one has worked harder to bring manufacturing jobs back to our shores, close unfair tax loopholes, and ensure that everyone plays by the same set of rules. As chairman of the Armed Services Committee, Senator Levin is a true champion for all those who serve, and his tireless work will be missed not just in his home state of Michigan, but by military families across our country," Obama added.

"Carl Levin's place in history belongs alongside Michigan's greatest statesmen," said Rep. Gary Peters (D-Mich.), who called Levin a mentor and a friend. "His leadership will be missed, but his legacy to Michigan and this country will outlive all of us.”

Levin's retirement gives Republicans renewed hope at picking up the seat, and a slightly easier path to winning control of the Senate.

The GOP needs to net six seats for control, and while Levin would have been a lock for reelection — he's won every election since 1990 by double digits — there's a chance Republicans could mount a serious push for the seat.

The campaign arm for Senate Republicans said the loss of Levin puts Democrats "back on their heels" ahead of the 2014 campaign.

"Over the last few months, the 2014 map has gone from sorta difficult to really tough for Senate Democrats," said Brad Dayspring, the communications director for the National Republican Senatorial Committee.

"Republicans can win in Michigan," Dayspring said.

The Democrats' preferred candidate for the race, according to sources in the state, is Rep. Gary Peters (D-Mich.).

Peters is a proven fundraiser and campaigner with close ties to the state's powerful labor unions as well as House Democratic leadership.

He won a tough suburban Detroit swing district in 2008 and held onto it in 2010 despite the GOP wave. Then, after his district was eliminated in redistricting, Peters defeated another incumbent Democrat in a primary for a heavily Democratic Detroit-based seat in 2012.

It's unclear who might run for the seat if Peters opts against a bid, as Democrats have a fairly short bench in the state.

On the GOP side, Republicans have a few potential candidates who could make the race competitive, though they also have short bench.

Both Reps. Mike Rogers (R-Mich.) and Candice Miller (R-Mich.) have proven they can win in swing districts and could be strong statewide candidates, but both now are House committee chairmen and might be less than eager to give up those roles. Rep. Justin AmashJustin AmashHouse votes to crack down on undocumented immigrants with gang ties GOP lawmaker taunts House conservatives: Trump’s base is not ‘small faction of obstructionists’ Overnight Finance: GOP plans to unveil tax framework in late September | Critical stretch for Trump tax team | Equifax CEO called to testify | Sanders unveils single-payer bill MORE (R-Mich.) is said to be considering a bid, though his strong libertarian streak may limit his general election appeal.

Rep. Dave Camp (R-Mich.) could also be interested in the seat, though it's may be difficult for him to give up his gavel of the House Ways & Means Committee. 

Democratic Senatorial Campaign Committee Chairman Michael BennetMichael Farrand BennetGOP eying 'blue slip' break to help Trump fill the courts NFL star claims he was victim of 'abusive conduct' by Las Vegas police Gardner throws support behind DREAM Act MORE (D-Colo.) predicted the seat would remain in Democratic hands.

"I am confident that we will recruit a great Democratic leader who will continue to fight for the values and priorities Senator Levin advocated for all these years," he said in a statement. "We fully expect to keep Michigan blue in November 2014.”

Other potential Democratic retirements include Sen. Tim JohnsonTim JohnsonCourt ruling could be game changer for Dems in Nevada Bank lobbyists counting down to Shelby’s exit Former GOP senator endorses Clinton after Orlando shooting MORE (D-S.D.), who many expect to leave the Senate, and Sen. Dick DurbinRichard (Dick) Joseph DurbinSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Overnight Defense: Senate passes 0B defense bill | 3,000 US troops heading to Afghanistan | Two more Navy officials fired over ship collisions Senate passes 0B defense bill MORE (D-Ill.), who has yet to make a public decision. Republican Sens. Mike JohannsMike JohannsLobbying World To buy a Swiss company, ChemChina must pass through Washington Republican senator vows to block nominees over ObamaCare co-ops MORE (Neb.) and Saxby ChamblissSaxby ChamblissFormer GOP senator: Let Dems engage on healthcare bill OPINION: Left-wing politics will be the demise of the Democratic Party GOP hopefuls crowd Georgia special race MORE (Ga.) have also announced they will retire.

— This story was last updated at 7:29 p.m.