MIAMI—Voters go to the polls Tuesday to decide whether a long-time legislator or a billionaire newcomer will be the Democratic nominee in one of the most closely watched and unusual U.S. Senate primaries in the nation.

U.S. Rep. Kendrick Meek is a political scion from Miami. Both President Barack Obama and former President Bill Clinton stumped for him last week. Jeff Greene is an entrepreneur who made his fortune partly by betting that the subprime mortgage bubble would burst, and who has spent an estimated $24 million of his own money blanketing the state with TV ads.

Meek, who is leading Greene by seven points in a Quinnipiac poll out last week, has gained momentum in the polls as he has described Greene as a meltdown mogul who is cozy with troubled professional boxer Mike Tyson, the best man at Greene's lavish wedding. Greene said he believed in redemption and even drove Tyson to rehab, which he also paid for.

For his part, Greene has attacked Meek as a failed career politician who is tainted by his association with a failed contractor who sought federal money and hired his mother, former U.S. Rep. Carrie Meek, as a consultant. Kendrick Meek said he and Carrie Meek acted appropriately, and has questioned the strategy of criticizing a rival's mother.

Whoever wins Tuesday's contest not only has to reunite the party after a bruising primary, but also must square off against two better-rested rivals: Gov. Charlie Crist, who is running as an independent, and former state House Speaker Marco Rubio, who faces only nominal opposition in Tuesday's GOP primary.

Neither Democrat fares well in a putative three-way race. Polls show Crist and Rubio in a dead heat and either Greene or Meek trailing by double digits.

Florida has been a recent harbinger of national sentiment, a swing state carried by George W. Bush in 2004 and Barack Obama in 2008 and where now the politics seem as disjointed as those across the nation.

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