Barack Obama, locked in a tight and contentious race with Hillary Rodham Clinton, was favored to win Tuesday's primary in Mississippi as the two Democrats battled for every last delegate in their historic contest for the party's presidential nomination. The Illinois senator campaigned in Mississippi on Monday, and mocked Clinton's suggestion that he could be her running mate. Saying he wanted to be "absolutely clear," he told supporters: "I don't want anybody here thinking that somehow, 'Well, you know, maybe I can get both.' Don't think that way. You have to make a choice in this election." "I am not running for vice president," Obama added. "I am running for president of the United States of America."