While's Mack's decision means one less prominent Republican will make a bid for the seat, the prospect of a crowded and competitive primary remains. State Senate President Mike Haridopolos (R) entered the race in in January, and enjoyed early fundraising success. But of late, Haridopolos has been surrounded by negative headlines including the revelation that he received over $150,000 in taxpayer money to write a book. Only a single copy of the text was initially published. Former state House Majority Leader Adam Hasner (R) has already formed an exploratory committee. Hasner, unlike many Republicans in the state was an early supporter of Sen. Marco Rubio (R-Fla.), which could provide him with a leg up in the race. While Rubio has said he will not be making an endorsement in the primary, GOP candidates are likely to highlight their own similarities and connections to the popular first-term senator, and Hasner may be in position to make the most credible argument on that front. Also looking at the race: former Sen. George LeMieux (R). Mack told the Times he has not made a decision about an endorsement in the GOP race. Nelson remains vulnerable to a strong Republican challenge in 2012. A Mason-Dixon poll conducted last month showed Nelson receiving less than 50 percent of the vote against all prospective challengers. Meanwhile, Vice President Joe Biden was in the state this week to help Nelson raise money. Bill Clinton was in Florida in January to fundraise for Nelson.