He tried being a Republican for a couple of decades, they let him get as high as he was going to get with the Governorship. He tried to push them for more, and they gave him the "nah". So now he moves over to the Democrats, an he'll milk them for a bit. In the twilight of his years, he'll be a hero to them. The embodiment of the red blooded macho male, who's finally seen the light.
This is just another way to stay in the limelight, and a giant fuck you to the Republicans for not backing him when he was eyeing the Presidency.