His lead in North Carolina is dwindling, but he's closing in on Senator Clinton in superdelegates.
She needs to win nearly all the remaining contests, analysts say, and persuade superdelegates that she has a better chance than Obama of beating John McCain.
His large margins of victory in Tuesday's contests put Clinton on the ropes.
McCain leads by 19 points nationally, but Romney could benefit from anti-McCain votes.
The two candidates are running neck and neck before the Republican primary Tuesday.