In Florida, Republicans describe Nelson as one of the luckiest politicians, with support they insist is a mile wide but only an inch deep. They point to his less-than-formidable opponents in past elections, former Rep. Bill McCollum and divisive 2000 recount figure Katherine Harris, whom he beat 60-38 percent in 2006.
Mack, in the first rush of fundraising, emerged with just $1.38 million cash on hand at the end of March, according to Federal Election Commission reports. LeMieux had $1.19 million. By comparison, Nelson reported $9.54 million cash on hand to run in a state with 10 expensive media markets.
This spring, Republicans, including freshman Sen. Marco Rubio, had talked up the candidacy of Florida's chief financial officer, Jeff Atwater, but he decided against running.
LeMieux claims Mack — great-grandson of baseball Hall of Fame manager Connie Mack and son of the former senator — is in Washington on name recognition only.
Mack "doesn't have the competency or character to be U.S. senator. If his name were Connie Smith, he wouldn't have been elected to anything," LeMieux said in an interview. LeMieux, who served as interim senator, added, "It's hard to ask for a promotion when you're not showing up for work."
Mack has the backing of likely Republican presidential nominee Mitt Romney and is considered the front-runner in the Aug. 14 primary. He's answering LeMieux's attacks with his own assault.
Seizing on published reports last week, Mack's campaign called for a Justice Department investigation into allegations that LeMieux pressured then-Gov. Charlie Crist to appoint him to the Senate for the remaining 16 months of Republican Mel Martinez's term in 2009. The newspaper reports suggested the two had a quid pro quo, with LeMieux backing Crist's unsuccessful Senate bid in 2010.
Mack shrugs off LeMieux's criticism as "juvenile" and insists that the former senator should be ashamed.
"George LeMieux is fixated on making a joke of his own campaign. We're fine to let him do that," Mack said in an interview, arguing that he's more focused on addressing the nation's problems.
Stepping into this scorched earth race is former Rep. Dave Weldon, who launched his late bid arguing that none of the candidates, including businessman Mike McCalister, has won over conservatives.
The races are certain to tighten as the November election closes in, especially with a divided electorate, outside money in the millions, and Obama and Romney spending millions more.
"Keep your eye on Florida because it is ground zero," Nelson said in an interview, pointing out that Florida's 29 electoral votes make it the biggest prize among swing states.
The U.S. Chamber of Commerce recently spent $2 million on an ad criticizing Nelson for his support for Obama's health care law.
"Obamacare will be a nightmare for seniors," the ad says. "Did Bill Nelson consider the consequences when he cast the deciding vote for Obamacare?"
Among several locations, the ads ran in Tallahassee and Panama City, part of the Florida Panhandle in the northwestern part of the state. The choice was intentional as state Republicans and Democrats say the folksy Nelson runs stronger in the Panhandle than most Democrats, including Obama.
Mack is already trying to undercut that support, referring to Obama and Nelson as "two lock-step liberals who are joined at the hip."
Nelson will benefit, though, from the Obama campaign's strong organization in the state, said Orlando Mayor Buddy Dyer, a Democrat who recently was re-elected.
"They've been at it for a year already in central Florida. ... They have a very visible presence," Dyer said.
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