He could rise through the ranks of SAG like Reagan did and maybe play a President or two in a well-received film. Also, he would have to come across as mainstream politically - no being out there in either direction. America voted for a Reagan once and probably wouldn't vote in another hardline supply sider; on the other hand, if he's seen as a Hollywood liberal, he won't make it out of the primaries.
Also, if he ends up as a governor first, say, of a moderate state, it could really help his cause.