Barack Obama, locked in a tight and contentious race with Hillary Rodham Clinton, won the March 11 primary in Mississippi, as they head into the next critical showdown in Pennsylvania on April 22.

Obama defeated Clinton in a minor contest in Wyoming on March 8 and retains the all-important overall delegate lead.

While campaigning in Mississippi on March 10, the Illinois senator mocked Clinton's suggestion that he could be her running mate.

"I am not running for vice president," Obama told supporters. "I am running for president of the United States of America."