With so much focus on the ever-changing race for the GOP presidential nomination, it's been easy to forget that the Democrats' slim majority in the US Senate may be slipping away. The chance of slippage got a little stronger today with news that Senator Ben Nelson (D-Nebraska) will announce that he will not seek re-election in 2012.

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Senator Nelson's seat had already been targeted by Republican groups as one that could help the GOP take control of the Senate next year. An anti-Nelson ad campaign had already begun and despite the Democratic Senatorial Campaign Committee spending over one million dollars on pro-Nelson ads, the senator has decided to ignore the pleas of his party's leadership and get out before he is voted out.

Despite being one of the most conservative Democrats in the Senate, Mr. Nelson still voted for President Barack Obama's extremely unpopular and obscenely expensive health care reform legislation earning him severe criticism in Nebraska. He also earned national infamy for negotiating what's become known as the Cornhusker Kickback - a massive legislative payoff in exchange for his pro-Obamacare vote.

Senator Nelson's seat is one of 33 up for grabs in 2012 and 23 of them are currently held by Democrats and the two Independents who caucus with them putting the Democrats' 53-47 majority at risk. With 6 other Democrats also choosing retirement over tough re-election fights, that risk is getting higher every day.

UPDATE: Senator Nelson has officially announced he will not seek re-election. "Simply put: It is time to move on," he said in a statement.