Bill, let's not forget mayors, governors, Congressional representatives, etc.
Clint Eastwood's entry level political position: Mayor of Carmel, CA (IIRC)
Sonny Bono's entry level political position: Representative to US House of Representatives
Fred Thompson's entry level political position: US Senator IIRC
Ronald Reagan's entry level political position: Governor of California
Jesse Ventura's entry level political position: Governor of Minnesota
Arnold Schwarzenegger's entry level political position: Govern(at)or of California

There have been several more.    And some former sports stars have also gone straight to Congress or governorships too.

If Evan Bayh can deliberately try to either hand his Senatorial seat to the Republicans or at least make sure the progressive/liberal Democrats have no shot at that seat - by dropping out with no warning, just a few days before the filing deadline for the Indiana primaries - why the heck shouldn't someone like Mellencamp play his own trump card, namely his wealth, name recognition, and popularity?