FORT LAUDERDALE, Florida (Reuters) - A senior adviser to Republican presidential candidate John McCain said on Tuesday that he was stepping down to keep a commitment he made not to campaign against Democrat Barack Obama. Mark McKinnon, who was in charge of the McCain campaign's advertising message, said he was still backing the Arizona senator, but that he was simply moving from active campaign participant to cheerleader. "I'll still be around occasionally in my lucky hat," said McKinnon, who often wears a distinctive hat. McKinnon, who was a key aide in President George W. Bush's two election victories, has expressed...