(CNN) -- Democratic Sen. Ben Nelson of Nebraska announced Tuesday that he will not run for re-election next year.
Nelson, 70, said in a statement that it was time to "step away from elective office, spend more time with my family, and look for ways to serve our state and nation."
"Simply put: It is time to move on," Nelson said in the statement.
Despite his record as a conservative Democrat who sometimes voted against his party, Nelson had been considered a top target of Republicans heading into the 2012 elections.
A former two-term governor of Nebraska, Nelson narrowly won election to the Senate in 2000 and easily won re-election in 2006.
His departure increases the difficulty for Democrats to retain their Senate majority next year.
Democrats currently have a 53-47 margin in the Senate, including two independents who caucus with them. In the 2012 elections, 23 of the 33 seats up for grabs are currently held by Democrats and the two independents.
So far, six other Democratic senators and two Republicans have announced they will retire instead of running for re-election next year.
A Senate Democratic leadership aide, speaking on condition of not being identified, told CNN that party officials are reaching out to former Nebraska Sen. Bob Kerrey to possibly run for Nelson's seat. Kerrey, 68, is a Medal of Honor winner from the Vietnam War who, like Nelson, served as both a Democratic governor and senator.
Nelson was known for looking out for his state's interests, especially on agriculture issues. He sided with Republicans on some high-profile issues, opposing the Supreme Court nomination of Elena Kagan and a climate-change measure pushed by the White House and Democrats.
President Barack Obama commended Nelson for working with both parties during his career, saying in a statement it was "a trait far too often overlooked in today's politics."
Jim Manley, the former long-time spokesman for Senate Majority Leader Harry Reid, D-Nevada, said Nelson was often a "thorn in Harry Reid's side." However, Manley said Reid "always respected the fact that Nelson was trying to do what was best for Nebraska."
According to the Senate Democratic leadership aide, Reid and Sen. Chuck Schumer, D-New York, tried to convince Nelson to stay in Congress after Nelson told a local newspaper a few weeks ago he was still deciding on whether to run.
"Over the last several weeks, there were intense conversations with Reid and Schumer trying to make Nelson understand how needed he was and how valued he is as a member of the caucus," the Senate aide said. "Ultimately they understood that this is a personal decision."
A Democratic strategist who spoke on condition of not being identified characterized Nelson's decision as disloyal to the party.
"He typically takes the easy way out," the strategist said. "The party has stood by him, but when the going gets tough, he abandons the party."
In his statement, Nelson called for "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."
"Public office is a place for public service, not personal profit," Nelson's statement said. "It's about promoting the common good, not the agenda of the radical right or the radical left. It's about fairness for all, not privileges for the few. And, it's about protecting the rights of individuals, even if it angers the majority. I hope and believe I have fulfilled these principles to the best of my ability."