Democratic leaders from the White House and Capitol Hill pleaded with Sen. Ben Nelson (D-Neb.), asking him to run for re-election for one main reason: the party is desperate to keep its Senate majority and it has no one else to run in Nebraska.

As is often the case, Nelson is letting his party down.

Democratic Sen. Ben Nelson of Nebraska will announce today that he is retiring after two terms, a serious blow to Democratic efforts to hold onto their majority in the chamber next November.

Nelson is scheduled to hold a press conference back home in Nebraska as early as today to make his decision official, said several Democratic insiders close to the leadership.

The 70-year-old Nelson was considered one of the most endangered Democratic incumbents this cycle. GOP-affiliated outside groups have already dumped hundreds of thousands of dollars into TV ads bashing Nelson, while the Democratic Senatorial Campaign Committee spent over $1 million on their own ad blitz to bolster his image.

That last part is of particular interest. Remember, Nelson waited until after Democratic and allied groups had invested quite a bit of money to strengthen his standing in Nebraska, and then decided to retire.

Also note, Nelson has voted with the right many times over the last couple of years — even on filibusters — offering Republicans cover on a wide range of issues. When pressed, Nelson would often tell his Democratic allies the votes were necessary to bolster his re-election bid. Now that he’s leaving, Nelson’s votes with Republicans appear to have no value at all.

That said, Nelson’s record wasn’t all bad. In 2009, after nearly balking many times, he stuck with his party on the Affordable Care Act and the Recovery Act. Those proved critical, and both measures would have failed without his vote. As exasperating as Nelson has been, he never drifted so far to the right that he was more conservative than Republicans.

In any case, what happens now? The Republican race appears to be coming down to state Attorney General Jon Bruning and state Treasurer Don Stenberg, both of whom are competing to appear more unhinged than the other. There isn’t much of a Democratic bench in Nebraska, though former Sen. Bob Kerrey (D), who retired 11 years ago, has reportedly expressed interest in a possible comeback.

There’s also former Sen. Chuck Hagel, who became an Obama ally and critic of the Republican Party, despite being a conservative.

While this sorts itself out, here’s a bottom line to remember: Ben Nelson just made it much tougher for Democrats to maintain a Senate majority in 2013.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.