LINCOLN, Neb. - Sen. Ben Nelson of Nebraska said Tuesday that he would retire rather than seek a third term next year, dealing a significant setback to Democratic efforts to keep control of the chamber.

The 70-year-old conservative Democrat, whose seat is being heavily targeted by Republicans in 2012, said in a statement that "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," he said.

Republicans, who need to net four seats to take back the Senate next year, say Nebraska has tilted further to the right in recent years. Nelson is the lone Democrat among the state's five-member congressional delegation, and the Republicans think Nelson's vote for President Obama's signature health-care legislation would have weighed him down.

Nelson still could have given Democrats a fighting chance. A two-term governor before winning a Senate seat, he has shown an ability to rebound after being down in previous statewide races.

But he recently expressed dismay about a divided Congress' inability to pass meaningful legislation, frustration that echoed in the Tuesday statement in which he said public office is "about promoting the common good, not the agenda of the radical right or the radical left."

Even as Nelson wavered on whether to seek another term, he piled up campaign cash, hired a campaign manager, and watched his party spend more than $1 million on ads supporting him. Prominent Nebraska Democrats said they were shocked by his announcement and said they were concerned about who they might field against the GOP nominee.

"I'm absolutely stunned," said Kathleen Fahey, a Democratic superdelegate in 2008. "Ben has been such a great senator for everybody. I'm not liking this."

The campaign preparation had left him with a healthy cash edge. He had more than $3 million last month, about twice his nearest rival, and had the luxury of stockpiling cash while Republicans focused on a crowded primary that includes Don Stenberg, the state's treasurer; Jon Bruning, the attorney general; Deb Fischer, a state senator; and Pat Flynn, an investment adviser.

Nelson was first elected to the Senate in 2000, defeating Stenberg to replace the retired Bob Kerrey. His centrist stance helped him get reelected over the years.