During a conference call to announce Missouri Sen. Claire McCaskill's endorsement, the junior senator from Illinois criticized Sen. Hillary Clinton, D-N.Y., for saying his campaign was trying to argue she was somehow disrespectful in what she said about King and President Lyndon Johnson the day before the New Hampshire primary, the Chicago Tribune reported.

"This is, you know, fascinating to me. I think what we saw this morning is why the American people are tired of Washington politicians and the games they play," Obama said. "Look, Sen. Clinton made an unfortunate remark, an ill-advised remark, about Dr. King and Lyndon Johnson … I think, offended some folks who felt that it somehow diminished King's role in bringing about the Civil Rights Act."

McCaskill, who was elected to the Senate in 2006, said she is backing him because he has proven he can work with both sides of the political aisle.

"As I worked with him trying to find a way to get past the partisan food fight, I realized this was an extraordinary leader," she said.