A top Obama administration adviser Sunday cast doubt on the legitimacy of Alvin Greene's Senate Democratic primary win South Carolina.

Asked if Greene's victory was legitimate, Senior White House Adviser David Axelrod said, "It doesn't appear so to me. The whole thing is odd. I don't know how really to explain it. I don't think anyone else does either."

Axelrod's comments come after House Majority Whip Jim Clyburn (D-S.C.) suggested last week that Greene's candidacy was a "plant" and called for an investigation.

"How he won the primary is a big mystery. Until you resolve that I don't see how you can claim to be a strong candidate," Axelrod said on NBC's "Meet the Press."

Greene is an unemployed military veteran. The Associated Press reported last week that Greene faces a felony pornography charge.