Updated at 3:45 p.m. with Nelson’s and Kerrey’s remarks

Sen. Ben Nelson (D-Neb.) announced Tuesday he will not seek reelection, leaving his seat as a strong pickup opportunity for the GOP in the 2012 election.

Nelson, 70, said in a web video that he wants to spend more time with his family and look for other ways to serve.

“Therefore, I am announcing today that I will not seek reelection,” he said. “Simply put: It is time to move on.”

The national Democratic Party had spent more than $1 million in advertising this year driving up Nelson’s personal approval rating, perhaps in hopes of convincing him that he could win in a dark red state. But Republican-aligned groups also spent heavily trying to define the moderate Democrat as an enabler of President Obama, particularly because Nelson voted for Obama’s health care bill.

View Photo Gallery: The Democrat from Nebraska is announced Tuesday that he won’t seek reelection in 2012.

His reelection race was expected to be an uphill battle either way, but there also remain questions about the strength of the GOP field that is vying to run against him. The field is led by state Attorney General Jon Bruning and also includes state Sen. Deb Fischer and state Treasurer Don Stenberg, a favorite of the tea party.

There has been some talk in recent weeks that popular Gov. Dave Heineman (R) might also run, but he has shown little interest. Other possibilities, now that Nelson is out, include members of the state’s all-GOP congressional delegation, particularly Reps. Lee Terry and Jeff Fortenberry.

It was not clear who might replace Nelson on the Democratic side, as Democrats’ bench in Nebraska is pretty thin. But former senator Bob Kerrey has been mentioned and would instantly give his party a fighting chance in a state where Obama took just 42 percent of the vote.

Kerrey, who also flirted with a return to the Senate in 2008, said recently that it is “highly unlikely” he would replace Nelson on the ballot. And in a statement to The Fix, he wasn’t elaborating.

“Ben’s retirement is a huge loss for Nebraska,” Kerrey wrote from a vacation in India. “I am very sad he’s leaving. That is as far as I am going (right now).”

Businessman Scott Kleeb, the 2008 Democratic nominee in an open seat race against now-Sen. Mike Johanns (R), told The Fix on Tuesday that he “has no interest.”

Over the last decade, Nelson has carved out one of the most conservative records of any Democrat, often irritating the party’s liberal base by voting with Republicans.

But when it came to the health care bill, Nelson delivered what many saw as the deciding vote. In return, he also got some concessions, derogatorily referred to as the “Cornhusker Kickback.” Early GOP advertising efforts have focused heavily on this arrangement, which included exempting his home state from paying billions in Medicaid expansion costs.

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