The criticism is nothing new for Nelson, who has a knack for drawing weaker opponents — Bill McCollum in 2000 and Katherine Harris in 2006 — while his popularity numbers remain relatively low. As the only Democrat elected statewide in Florida, Nelson is the ultimate political survivor.

But Mack is waging a far more aggressive campaign than Nelson’s prior opponents. The congressman, noting the two-term Nelson’s voting record, has tried to frame the incumbent as a “lockstep liberal.”

Voters, however, have drawn a distinction between Nelson and President Obama, who’s essentially tied with Republican Mitt Romney while Nelson nurses a comfortable lead.

Nelson’s secret: He has shied away from Obama at times, said he disagreed with the president and has championed causes like stopping pythons in the Everglades or ensuring that BP adequately compensated Gulf Coast residents after the oil spill.

Though Nelson voted against the bank bailout, he did provide crucial support for the stimulus and the president’s healthcare bill, which narrowly passed. Mack has made those votes central to his attack on Nelson’s record.

In voting for Obamacare, Republicans note, Democrats like Nelson signed off on future reductions to Medicare of more than $700 billion.

Nelson notes that Mack voted for vice-presidential candidate Paul Ryan’s budget plans that vested the Obamacare cuts and would have reduced future Medicare benefits by turning the program into a voucher-like system that relies more on private health-insurance companies.

Mack’s campaign on Wednesday issued a written statement from a surrogate, former Republican candidate Mike McCalister, that criticized Nelson for voting to cut the defense budget and raise the debt ceiling.

“As I travel around Florida,” McCalister said, “I can assure Bill Nelson that he can no longer hide behind his fraudulent moderate mask.”

But Nelson said Mack is the one hiding his identity because many voters think he’s his father, Connie Mack III, a senator-turned-lobbyist. Still, this could be Nelson’s toughest race for Senate yet, especially as outside third-party groups dump money into Florida on negative ads attacking the Democrat.

When asked how tough the race will be, Nelson gave atypically low-key answer.

“I don’t know,” he said. “I’ll have to let you know closer to the election.”