Sen. Ben Nelson (D-Neb.) has decided against running for reelection, dealing a serious blow to Democratic hopes of retaining the Senate.
Nelson, a centrist Democrat who had been a thorn in the side of liberals for years, faced a difficult race in a solidly red state.
In a statement, Nelson said it was time for him to step aside.
"There is much more that needs to be done to keep America strong," he said in a statement to supporters. "And while I relish the opportunity to undertake the work that lies ahead, I also feel it’s time for me to step away from elective office, spend more time with my family, and look for new ways to serve our state and nation.
"Therefore, I am announcing today that I will not seek reelection. Simply put: It is time to move on."
There is little doubt that Nelson's decision hurts Democrats' chances of holding the Senate.
Democrats have a very short bench in the Cornhusker State.
Former Sen. Bob Kerrey (D-Neb.), who retired in 2001, is likely the only Democrat who could make this a competitive race. While he hasn’t closed the door on a run, he called it “highly unlikely” that he would jump in, telling the Nebraska Watchdog earlier this month that “It’s not what I would consider being my logical career path.”
Several Republicans are already in the race, including state Attorney General Jon Bruning, state Sen. Deb Fischer and state Treasurer Don Stenberg.
And Nebraska Gov. Dave Heineman (R) has said that national Republicans had encouraged him to jump into the the contest.
Some liberals won't be sorry to see Nelson retire, given the centrist Nebraskan's history of opposing his party on high-profile issues. The Huffington Post's headline on his retirement story was "Bye Bye Ben. Human impediment to progress retiring from Senate."
Still, Nelson cast a critical vote in favor of healthcare reform in 2009, ensuring approval of President Obama's signature legislative achievement.
It ended up costing Nelson, who was seen as vulnerable next year.
Republican-affiliated outside spending groups, including American Crossroads, had spent more than a million attacking him for his support of Democrats' healthcare reform law, causing the Democratic Senatorial Campaign Committee to spend heavily on ads to defend Nelson.
Obama released a statement Tuesday evening praising Nelson's career.
"Over the course of his career, Ben’s commitment to working with both Democrats and Republicans across a broad range of issues is a trait far too often overlooked in today’s politics. Michelle and I commend Ben for his service, and wish him and his family well in the future," he said.
The two-term senator has $3 million in the bank, which could be used to help another Democrat in the race if he chooses to give it to the Democratic Senatorial Campaign Committee or the Nebraska Democratic Party.
Ever since the 2010 election that ushered in the Republican House, there have been doubts about whether Democrats could retain the Senate in 2012.
The party is defending 23 Senate seats to the Republicans' 10 in next year's election, and the GOP needs a net gain of only four seats — if Obama wins reelection — to take control of the upper chamber.
Democrats in recent weeks have felt better about their chances, given Obama's rising poll numbers and a legislative victory last week in the fight over extending the payroll tax credit.
In Massachusetts, Democrats have a strong candidate in liberal favorite Elizabeth Warren, who set up the new Consumer Financial Protection Bureau created by the Wall Street reform bill. Warren has been a solid fundraiser and gives Democrats a chance of gaining the seat held by GOP Sen. Scott Brown.
Sen. Charles Schumer (D-N.Y.), who leads messaging efforts for Senate Democrats, earlier this month told The Hill: “I think we’re very, very likely to keep the Senate and I think there’s a darn good chance we stay the same or pick up seats.”
But Democrats also have some difficult seats to defend. Even before Nelson's decision, the party faced a difficult challenge in retaining Sen. Kent Conrad's seat in North Dakota.
Democratic incumbents in Missouri, Montana, Ohio, Florida and Pennsylvania are also seen as vulnerable, and Republicans believe their party has a good shot at picking up open seats in Virginia and Wisconsin.
Nelson is the seventh member of the Democratic Caucus to announce his retirement, and his decision means Republicans can shift resources elsewhere and spend more targeting other vulnerable Senate Democrats this cycle, including Sens. Claire McCaskill (D-Mo.) and Jon Tester (D-Mont.).
Democratic Senatorial Campaign Committee Chairwoman Patty Murray (D-Wash.) argued Tuesday that a Democrat could still win Nelson's seat.
"Republicans will continue to have their hands full with a very divisive primary in the state, which will provide an opportunity for Democrats to remain competitive," she said in a statement. "We remain confident that we will hold the majority next year because incumbents have built strong campaign organizations in their states and we’ve recruited great candidates who are generating enthusiasm around the country."
— Daniel Strauss contributed to this story.
This story was originally posted at 1:33 p.m. and last updated at 4:55 p.m.