Longtime Sen. Carl Levin Carl Milton LevinMichigan to pay 0M to victims of Flint water crisis Unintended consequences of killing the filibuster Inspector general independence must be a bipartisan priority in 2020 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 Rockefeller John (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 Harkin Thomas (Tom) Richard HarkinThe Memo: Trump attacks on Harris risk backfiring Ernst challenges Greenfield to six debates in Iowa Senate race Biden unveils disability rights plan: 'Your voices must be heard' 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 Reed John (Jack) Francis ReedWhen 'Buy American' and common sense collide Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts 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 Amash Justin AmashOn The Trail: How Nancy Pelosi could improbably become president History is on Edward Snowden's side: Now it's time to give him a full pardon Trump says he's considering Snowden pardon 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 Bennet Michael Farrand BennetOVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Senate Democrats demand White House fire controversial head of public lands agency Next crisis, keep people working and give them raises 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 Johnson Timothy (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 Durbin Richard (Dick) Joseph DurbinTop GOP senator calls for Biden to release list of possible Supreme Court picks GOP ramps up attacks on Democrats over talk of nixing filibuster Catholic group launches .7M campaign against Biden targeting swing-state voters MORE (D-Ill.), who has yet to make a public decision. Republican Sens. Mike Johanns Michael (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 Chambliss Clarence (Saxby) Saxby ChamblissLobbying world GOP lobbyist tapped for White House legislative affairs The Hill's Morning Report - Gillibrand drops out as number of debaters shrinks MORE (Ga.) have also announced they will retire.

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