Longtime Sen. Carl LevinCarl Milton LevinOvernight Defense: First group of Afghan evacuees arrives in Virginia | Biden signs Capitol security funding bill, reimbursing Guard | Pentagon raises health protection level weeks after lowering it Biden pays tribute to late Sen. Levin: 'Embodied the best of who we are' Former Colorado Gov. Richard Lamm dead at 85 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 RockefellerJohn (Jay) Davison RockefellerBottom Line World Health Day: It's time to fight preventable disease Lobbying World MORE (D-W.Va.), Frank Lautenberg (D-N.J.) and Tom HarkinThomas (Tom) Richard HarkinBiden celebrates anniversary of Americans with Disabilities Act Ex-Rep. Abby Finkenauer running for Senate in Iowa We need a voting rights workaround 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 ReedHouse panel looks to help military sexual assault survivors Senate panel votes to make women register for draft Senators hail 'historic changes' as competing proposals to tackle military sexual assault advance 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 AmashAmash warns of turning lawmakers like Cheney into 'heroes' Cheney set to be face of anti-Trump GOP Biden: 'Prince Philip gladly dedicated himself to the people of the UK' 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 BennetLawmakers can't reconcile weakening the SALT cap with progressive goals How Sen. Graham can help fix the labor shortage with commonsense immigration reform For true American prosperity, make the child tax credit permanent 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 JohnsonTimothy (Tim) Peter JohnsonCornell to launch new bipartisan publication led by former Rep. Steve Israel Trump faces tough path to Fannie Mae, Freddie Mac overhaul Several hurt when truck runs into minimum wage protesters in Michigan MORE (D-S.D.), who many expect to leave the Senate, and Sen. Richard DurbinDick DurbinCongress should butt out of Supreme Court's business Inmates grapple with uncertainty over Biden prison plan Democrats warn shrinking Biden's spending plan could backfire MORE (D-Ill.), who has yet to make a public decision. Republican Sens. Mike JohannsMichael (Mike) Owen JohannsMeet the Democratic sleeper candidate gunning for Senate in Nebraska Farmers, tax incentives can ease the pain of a smaller farm bill Lobbying World MORE (Neb.) and Saxby ChamblissClarence (Saxby) Saxby ChamblissEffective and profitable climate solutions are within the nation's farms and forests Live coverage: Georgia Senate runoffs Trump, Biden face new head-to-head contest in Georgia MORE (Ga.) have also announced they will retire.

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