Sen. Ben Nelson (D-NE) is set to announce on Tuesday that he will retire in 2012, a Democratic source has confirmed to TPM.

Nelson was already probably the most vulnerable Democratic incumbent going into 2012, and Republicans and GOP-aligned groups were already advertising against him.

Holding this seat could be difficult for Democrats, as the state voted 57%-42% for John McCain in 2008, and has not voted Democratic for president since the 1964 Lyndon Johnson landslide.

Nelson’s retirement was first reported by Politico.

Considering he was more Republican than Democrat, I’m not too broken up by this. However, Nebraskans should be careful they don’t wind up with a Tea Party acolyte.