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No front-runner emerged from the pack of Republicans who were vying for the nomination for most of 2011. Mack surged to the top of the field when he announced in October, leading other candidates to drop out, but his surge soon faded, and recent polls have shown Nelson with a modest lead over the Republicans.

Mack faces former Sen. George LeMieux (R-Fla.) and Mike McCalister in the primary, but both have struggled to demonstrate growing momentum for their candidacies.

Nelson, meanwhile, has been raising money at a rapid pace, and has about a 7-to-1 cash advantage over both Mack and LeMieux as of the end of March. McCalister has even less on hand.