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The incumbent Nelson is still facing serious challenges to his reelection bid, with outside spending groups including American Crossroads and Freedom PAC, a super-PAC recently helped out by a large donation from casino mogul Sheldon Adelson, investing millions into helping his opposition. The Tampa Bay Times reported that more than $14 million has already been spent or reserved in air time in Florida.

Mack has trailed Nelson in all but one recent poll. A Rasmussen poll released Wednesday showed Mack up by 9 points (46 to 37 percent), but the RealClearPolitics tracking average puts Nelson ahead by just over 2 points. 

“With everyone’s focus now squarely on November, money is flowing into this campaign because its clear Bill NelsonBill NelsonTrump's Commerce pick admits to unknowingly hiring undocumented worker Senate Democrats brace for Trump era Senators introduce dueling miners bills MORE can and will be defeated. There is also no shortage of outside groups who are and who will continue to spend great sums of money to defeat Washington’s most notorious LockStep Liberal, Bill Nelson, as they should,” Mack’s campaign manager, Jeff Cohen, said in a statement.