Politico reports:

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.