Sen. Ben Nelson, D-Neb., will announce later on Tuesday that he will retire after two terms in office, according to a senior Democratic operative familiar with his plans. Nelson's decision is a serious blow to Democrats' hopes of maintaining its slim Senate majority, turning what would have been a competitive red-state Senate race into a very likely GOP pickup next year.
News of Nelson's retirement is especially tough for Democrats, given that outside groups poured over $1 million in ads on his behalf to shore up his standing in the state. The ads were also aired in hopes of convincing Nelson to run for a third term.
(RELATED: Sen. Ben Nelson's Almanac Profile)
Republicans have aggressively targeted Nelson this year, and outside GOP groups had already begun pouring money into the state, hitting him over his vote for President Obama's health care law.
The news of Nelson's retirement was first reported by Politico.
Nelson looked in decent shape for re-election, according to his campaign's own polling. In early December, Nelson released a poll showing his favorability was on the rise and that he improved his standing against Republican opponents. Democratic groups credited the ad campaign for improving his favorables.
But throughout the last month, Nelson continued to face a barrage of attacks from the opposition. Crossroads GPS decided to drop more than more than $500,000 against Nelson in the state's two largest media markets. Americans for Prosperity purchased $120,000 on cable television for anti-Nelson ads. And Nelson's likely Republican opponent, Attorney General Jon Bruning, launched an ad of his own earlier this month, in which he contrasts himself with Nelson.
Even with Nelson in the race, the senator was considered the most vulnerable Democrat up for re-election, given the conservative nature of his state. With Nelson out of the race, Republicans hold a clear edge here. The Democratic bench is weak in the state, though some operatives have discussed the possibility that former Sen. Bob Kerrey could run again.
Bruning is the frontrunner in the Republican primary, which also includes Treasurer Don Stenberg, state Sen. Deb Fischer, and businessman Pat Flynn. Bruning leads in fundraising and recent polls.
There was speculation Gov. Dave Heineman could still enter the Republican race, he had framed a potential run as something he would consider if he had to in order to turn the seat Republican -- something that wouldn't be necessary with Nelson taking a pass on reelection.
Nelson is the seventh Democrat - including Connecticut Sen. Joe Lieberman -- to announce retirement plans this cycle. Two Republicans have announced they are retiring, and former Sen. John Ensign resigned earlier this year.
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