RENO, Nev.—Harvey Whittemore rose from mopping floors as a student at the University of Nevada here to become one of the state’s top power brokers. Now he is entangled in litigation that is roiling the political landscape in one of the nation’s swing states.

Mr. Whittemore, 55 years old, worked his way to the top of Nevada’s political scene as a lobbyist and prodigious fundraiser over much of the past quarter-century. He has been an ally and donor to both of the state’s U.S. senators—Democratic Majority Leader Harry Reid and Republican Dean Heller.

In January, Mr. Whittemore was sued by …

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