OMAHA, Neb. (AP) -- Nebraska Democrat Ben Nelson says he decided to leave the U.S. Senate after 2012 because he wants more free time, not because he's retiring.
Nelson told The Associated Press on Wednesday that he's not sure what's next, but he's sure he'll have another opportunity to serve.
The 70-year-old centrist Democrat and former governor announced his decision not to seek another term Tuesday. His departure is expected to make it easier for Republicans to win the only seat in Nebraska's congressional delegation that hasn't already gone to the GOP.
But Nelson says he doesn't think it's too late for Democrats to run.
He says he's also confident he could have won himself, saying his poll numbers have improved despite being hammered by conservatives for supporting the federal health care overhaul.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
Democratic Sen. Ben Nelson survived nearly two decades representing heavily Republican Nebraska by carving a path down the political center. But faced with navigating that road in an increasingly polarizing climate, Nelson is stepping away - and swinging the door wide open for the GOP.
Nelson, the lone Democrat in Nebraska's five-member congressional delegation, announced Tuesday that he wouldn't seek a third term. He was facing a tough campaign against several Republicans who've spent the past several months attacking his support for President Barack Obama's health care overhaul and federal stimulus legislation.
"Who can blame him given the current political mood of the country?" said former state Democratic Party Chairman Steve Achelpohl.
Other Democrats lamented Nelson's decision to retire, fearing it sets up the GOP for an easy victory next year. Republicans need to net just four seats to reclaim control of the Senate, and Nebraska looks to be an easy pickup.
There are no Democrats in line to take Nelson's place in the increasingly conservative state. He joins several other Democrats to retire from the Senate, including Virginia's Jim Webb and North Dakota's Kent Conrad.
After months of speculation that he would leave office, the 70-year-old conservative Democrat told supporters in an emailed statement that he felt it was time he "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 re-election," he said. "Simply put: It is time to move on."
The former two-term governor thrived in Nebraska politics partly because he was willing to support Republican ideas when he believed they were in the state's best interests, Achelpohl said.
"Nebraskans are generally independent thinkers, and he was certainly an independent thinker," Achelpohl said. "He just had his finger right on the pulse of the predominant political thinking right in our state and nationally."
Democrats banking on Nelson's ability to leverage those centrist stances and capture statewide races were left scrambling, and many state activists acknowledged being taken by surprise.
While some floated the names of state Sen. Steve Lathrop of Omaha and Nelson's former lieutenant governor, Kim Robak, as possible contenders, many said it was too early to know who might run. Messages seeking comment were left for Lathrop and Robak.
A dream candidate for Democrats: former Nebraska Sen. Bob Kerrey. Traveling in India on Tuesday, Kerrey told The Washington Post, "Ben's retirement is a huge loss for Nebraska. I am very sad he's leaving. That is as far as I am going (right now)."
Democrats acknowledged the party will face a steep uphill fight to hold on to Nelson's seat, but pointed to a crowded GOP primary field with no obvious front-runner. The ticket includes Nebraska Attorney General Jon Bruning, state Treasurer Don Stenberg, state Sen. Deb Fischer, and investment adviser Pat Flynn.
"This virtually guarantees a Republican victory in 2012," said University of Nebraska Lincoln political scientist Mike Wagner. "There's almost no scenario in which a Democrat can win - especially at this late stage."
National Republican party leaders also have encouraged Gov. Dave Heineman to join the race, but Heineman has said it would take a lot to persuade him.
The Senate's Democratic campaign chairman, Sen. Patty Murray of Washington, said she expected that Republicans would "have their hands full with a very divisive primary in the state, which will provide an opportunity for Democrats to remain competitive."
State Sen. Heath Mello, an Omaha Democrat who worked as an aide to Nelson, said he feared Nelson's retirement would inject more partisan politics into an already heated race.
"That is not the way Nebraskans have chosen their senators in the past," Mello said. "We've always elected independent-minded people to represent Nebraska's interests, ahead of the political parties."
Nelson has recently expressed dismay about a divided Congress' inability to pass meaningful legislation, frustration that echoed in his statement Tuesday.
"I encourage those who will follow in my footsteps to look for common ground and to work together in bipartisan ways to do what's best for the country, not just one political party," he said.
Even as Nelson wavered about a re-election bid, he piled up campaign cash, hired a campaign manager and watched his party spend more than $1 million on ads supporting him. The preparation left him with more than $3 million campaign cash on hand last month, about twice his nearest competitor.
The Democrats' Majority PAC alone spent more than $406,000 on media buys and production costs for Nelson's expected re-election campaign in seven separate expenditures between Sept. 9 and Dec. 9.
"I'm absolutely stunned," Kathleen Fahey, a Democratic super-delegate in 2008, said of Nelson's announcement. "Ben has been such a great senator for everybody. I'm not liking this."
Nelson first was elected to the Senate in 2000, defeating Stenberg, a Republican and currently the state treasurer, to replace the retired Kerrey. Nelson positioned himself as a centrist supporting both Democratic and Republican legislation.
He was among only two Senate Democrats to support a failed GOP bid to block new federal controls on power plant pollution that blows downwind into other states earlier this year, and he took great pride in his membership in the 2005 "Gang of 14," made up of Republicans and Democrats who brokered a deal to avoid a filibuster showdown over President George W. Bush's judicial nominees.
However, Nelson's vote in favor of Obama's signature health reform legislation left the GOP confident that they could beat him next year. The health reforms are strongly opposed by many Nebraska conservatives, and after the vote Nebraska Republicans immediately kicked off a "Give Ben the Boot" campaign.
Nelson also was one of five Democratic senators targeted by a national conservative group with ties to GOP strategist Karl Rove. The group, Crossroads GPS, spent $1.6 million on ads attacking Nelson as well as Sens. Bill Nelson of Florida, Clair McCaskill of Missouri, Jon Tester of Montana and Sherrod Brown of Ohio - all considered top targets by national Republicans in 2012.
"For once Senator Nelson has listened to Nebraskans," Nebraska Republican Party Chairman Mark Fahleson said Tuesday. "The Nebraska Republican Party is more focused than ever on electing another conservative Republican to join Sen. Mike Johanns and recapturing the U.S. Senate so that we can reverse the damage done by Ben Nelson, Washington Democrats and the Obama Administration."
Nelson upset incumbent Nebraska Gov. Kay Orr in 1990 to earn his first statewide office and was re-elected governor in 1994 by a landslide. In 1996, he reneged on a campaign pledge that he wouldn't seek higher office while governor and announced his candidacy for the Senate seat vacated by the retiring Sen. Jim Exon.
Omaha millionaire businessman Chuck Hagel soundly defeated Nelson in that Senate race, but the two later served as colleagues when Nelson was elected in 2000.
Bruning on Tuesday wished Nelson well and praised him as "a dedicated public servant of the state of Nebraska for over two decades," while Fischer expressed confidence the GOP would now claim the seat.
"I think we have a strong group of Republicans, and I happen to believe the seat will go to a Republican," she said.
Stenberg thanked Nelson for his service, but said Nebraskans need "a genuine, lifelong conservative."
Associated Press writer Larry Margasak in Washington and Grant Schulte in Lincoln, Neb., contributed to this report.