The Siegelman indictment and federalism

Don Siegelman and his cohorts are being prosecuted under the federal Racketeer-Influenced and Corrupt Organizations (RICO) Act. The indictment alleges that while Don Siegelman was Governor, the entire executive branch of Alabama's government constituted an "enterprise," as defined by the RICO statute, that was engaged in a pattern of racketeering activity.

Ever since I read the indictment, one thing has bothered me. Why is the federal government involved in prosecuting the corrupt practices of state and local government officials? I am all for eliminating public corruption, but I'm not convinced that it's the job of the federal government to prosecute it when its impact is limited to a particular state.

The U.S. Constitution gives the federal government the power to "regulate interstate commerce." Congress enacted the RICO statute in order to combat organized crime, which is often national or regional in scope. I don't doubt that Don Siegelman, et. al. engaged in corrupt practices, but it seems to me that any relation of their activities to interstate commerce is only incidental.

That leads me to a few questions: How do the feds claim jurisdiction here? More specifically, how have the courts viewed the application of RICO to corrupt state and local government officials, as opposed to organized crime bosses, the Mafia, etc.? Even if the case against Siegelman is legit...is notoriously difficult to prove a conspiracy of any type. Why do the prosecutors think that this will be any different?