Barack Obama is trying to win over superdelegates and says he's not taking the Democratic presidential nomination for granted.

Obama created a stir Thursday in an unannounced visit during House votes that lasted more than half an hour. He was surrounded by well-wishers calling him, "Mr. President" and reaching out to pat him on the back.

The Illinois senator told reporters afterward that he was asking members of Congress to support his White House candidacy. He said he wants to bring the party together behind him as soon as possible, but he recognizes he'll have to keep on working to finally defeat Hillary Rodham Clinton.

He said Clinton is a formidable candidate who is likely to win West Virginia and Kentucky, which vote in the next two weeks.