Longtime Sen. Carl LevinCarl LevinObama to preserve torture report in presidential papers 'Nuclear option' for Supreme Court nominees will damage Senate McCain's Supreme Court strategy leads to nuclear Senate 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 RockefellerObama to preserve torture report in presidential papers Lobbying world Overnight Tech: Senators place holds on FCC commissioner MORE (D-W.Va.), Frank Lautenberg (D-N.J.) and Tom HarkinTom HarkinGrassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream Do candidates care about our health or just how much it costs? 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 ReedJack ReedSenate panel easily approves waiver for Mattis Live coverage: Mattis confirmation hearing for Pentagon Democrats are playing with fire on Russia 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 AmashGOP lawmaker on Trump's Lewis tweets: 'Dude, just stop' House passes Mattis waiver, setting up quick confirmation House takes first step to repeal ObamaCare 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 BennetCorrected — Lawmakers: Trump can't stop investigation of Clinton email case Cory Booker kicks off 2020 maneuvering in the Senate Dems to GOP leadership: Help us to fix ObamaCare 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. Richard DurbinDick DurbinCubs celebrate World Series win at White House HUD finalizes rule to protect children from lead Trump should work with Congress to save 'Dreamers' MORE (D-Ill.), who has yet to make a public decision. Republican Sens. Mike JohannsMike JohannsTo buy a Swiss company, ChemChina must pass through Washington Republican senator vows to block nominees over ObamaCare co-ops Revisiting insurance regulatory reform in a post-crisis world MORE (Neb.) and Saxby ChamblissSaxby ChamblissWyden hammers CIA chief over Senate spying Cruz is a liability Inside Paul Ryan’s brain trust MORE (Ga.) have also announced they will retire.

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