Sen. Ben Nelson (D-Neb.) answers questions this month on Capitol Hill. (Alex Wong, Getty Images )
Reporting from Washington — Nebraska Sen. Ben Nelson announced Tuesday that he would not seek a third term in 2012, another setback for Democrats in their effort to retain control of the Senate.
Nelson, the most conservative Democrat in the party's 53-member caucus, also was considered one of the party's most endangered incumbents. He had indicated early in the year that he was prepared for the fight to retain his seat. But the steady brinkmanship that came to mark the legislative battles of 2011 led him to reevaluate his interest in remaining in Washington, according to aides.
"Simply put, it is time to move on," Nelson said in a video message announcing his decision. "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."
Although the Nebraskan often voted with Republicans, he did side with his party on key issues, most notably in the 2010 vote to pass President Obama's healthcare law.
He became a target for opponents of the landmark bill because of what became known as the "Cornhusker Kickback" — a provision intended to secure his vote by boosting federal Medicaid funding to the state. That controversial provision was quickly repealed.
Crossroads GPS, a conservative advocacy group, launched a six-figure ad buy in Nebraska this month arguing that Nelson "sold out to Obama when it mattered most."
Nelson is the seventh member of the Democratic caucus to announce his retirement. The party already faced an uphill climb in the 2012 congressional races, when it will have to defend 23 of the 33 seats up for reelection and has few clear opportunities to take Republican-held seats.
Nelson had served two terms as Nebraska's governor before he was elected to the Senate in 2000. He was reelected in 2006, a strong year for Democrats nationally.
The party already had spent more than $1 million on television advertising featuring Nelson.
Washington Sen. Patty Murray, chairwoman of the Democratic Senatorial Campaign Committee, promised in a statement only that the party would be "competitive" in the race without Nelson running.
In a statement, Obama praised Nelson's decades of public service and said his "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."
Several Republicans had already lined up to challenge him, including the state's attorney general and treasurer.
"Sen. Nelson recognized that his support for President Obama's reckless tax-and-spend agenda left him in a grave political situation," said Brian Walsh, a spokesman for the National Republican Senatorial Committee.
Democrats floated former Sen. Bob Kerrey as a possible replacement. Nelson replaced Kerrey in 2001. But Kerrey, also a former governor, has not been a presence in Nebraska in years, having served for a decade as president of the New School in New York.
"Nelson was only a 50-50 bet for reelection, and that's putting a good face on it," said Jennifer Duffy, who analyzes Senate races for the nonpartisan Cook Political Report. "Now the seat is just out of their reach."
Nelson has $3 million left in his campaign war chest that can be tapped by the Democratic nominee.