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FLORIDA: Democratic Senator Bill Nelson (left) will face Republican Representative Connie Mack in what is expected to be one of the country’s most expensive races.

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INDIANA - Democrat Joe Donnelly (left), a congressman, will face Republican Richard Mourdock, a state treasurer who defeated incumbent Senator Richard Lugar in the GOP primary.

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MASSACHUSETTS - Republican incumbent Scott Brown, who won the seat in a special election two years ago, will face Elizabeth Warren in the country's most expensive Senate race.

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MISSOURI - Democratic Senator Claire McCaskill will face Republican Representative Todd Akin, who made national headlines for his comments about rape.

Left: Justin Sullivan/Getty Images; Right: AP Photo

MONTANA - Democratic Senator Jon Tester (left) faces a considerable challenge against GOP Representative Denny Rehberg.

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NEVADA - Senator Dean Heller, who took the seat after fellow Republican John Ensign resigned, is facing Democratic Representative Shelley Berkley.

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NORTH DAKOTA - Republican Representative Rick Berg will face Democrat and former state attorney general Heidi Heitkamp for the seat being vacated by retiring Democratic Senator Kent Conrad.

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VIRGINIA - Two former governors face off with Republican George Allen (left) and Democrat Tim Kaine. Allen is trying to regain the seat he lost six years ago to now-retiring Senator Jim Webb.

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WISCONSIN - Republican former governor Tommy Thompson will go against Democratic Representative Tammy Baldwin for the Senate seat.