Nelson, the lone Democrat in Nebraska's five-member congressional delegation, faced a tough re-election campaign against a large group of Republican challengers who have spent the past several months attacking his support for President Barack Obama's health care overhaul and federal stimulus legislation.

Republicans must net four seats to take back the Senate in 2012, and Nebraska now 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 e-mailed statement Tuesday 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," said Nelson, a former two-term governor. "Simply put: It is time to move on."

Democrats banking on Nelson's ability to leverage his centrist stances and capture statewide races were left scrambling; 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 is 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 battle to hold on to Nelson's seat, but pointed to a crowded Republican primary field with no obvious front-runner. The contenders include 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 to run.

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."

A two-term governor before winning a Senate seat, 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.