Senator Ben Nelson (D-NE) has scheduled a press conference for later Tuesday to announce that he is retiring from Congress, according to a published report.

Nelson, 70, has been at the epicenter of major ad buys by both parties and outside groups, and he's become seen as one of the most endangered Democrats in the Senate. A total of 23 seats held by Democrats are up for reelection in 2012, while just 10 Republican Senators will be defending their jobs.

Because of his potentially key status for Democrats, top party officials have been lobbying Nelson to stay in the race for one more term. He's suggested for weeks now that a decision on his political future would be in the cards before year's end.

News that Nelson plans to quit was first picked up by Politico.

It's not clear who Democrats would get to run for Nelson's seat. Some have suggested that former Nebraska Gov. Bob Kerrey, also a former Senator, could be their leading candidate.

Kerrey has refused to say whether he would jump in the race, but hasn't ruled it out either.