Mack is the frontrunner in the Republican race. Polling, name ID, money and just about every other metric that matters indicates as much. But he's not running against nobodies. His opponents are a former senator (George LeMieux) and a former congressman (Dave Weldon). Rejecting a debate invitation out of hand isn't going to win Mack any good will in Republican circles. And it shows why the party has for months been searching for a viable Mack-alternative.
Yet the Gulf Coast congressman may well be his party's nominee and may soon get his chance to debate Nelson. But if he hopes to rally Republicans behind his cause in the fall, a premature coronation ceremony weeks before a single vote has been cast in the primary isn't a very good idea.