"Sheltrown says that he realized in order to run a successful race, he would have had to abandon his duties as a legislator to campaign full-time. He cannot run again for state House because of term limits."

He didn't mention He would be fighting a machine that already has the financing available, and the right connections.  Political insiders suggest it was the realization that Gary McDowell, current State Rep from Rudyard is Stupak's hand picked successor, and will likely receive support from team Stupak, and the Michigan Democrat Party.  With rumors of more than $2 million in that war chest, it would be a formidable task to go against it.