Clinton campaign chairman Terry McAuliffe said in a statement: "We respect John Edwards, but as the voters of West Virginia showed last night, this thing is far from over."

Political strategist and Clinton ally James Carville said Edwards' endorse was a psychological boost for Obama, but unlikely to sway many voters.

"I think it certainly helps in terms of the psychology of the superdelegates," Carville told ABC's "Good Morning America" on Thursday, referring to the elected officials and party leaders who will ultimately determine the Democratic nominee.

A person close to Edwards, speaking on condition of anonymity, said he wanted to get involved now to begin unifying the party. Edwards and Obama spoke by phone Tuesday night, and Edwards agreed to fly to Grand Rapids the next day.

Edwards didn't even tell many of his former top advisers of his decision because he wanted to inform Clinton personally, said the person close to him. His wife, Elizabeth, who has said she thinks Clinton has the superior health care plan, did not accompany him and is not part of the endorsement.