The Comics Journal once described Miller's 80s/early 90s politics as "mugged liberal," which I think was once apt. He satirized Reagan in The Dark Knight Returns; his portrayals of corruption in Daredevil and Batman: Year One were boilerplate old-school liberal along the lines of Jimmy Breslin; and Give Me Liberty, despite its Objectivist influences, is pretty rough on conservatives and corporate hegemony. In one of the storylines, he has a Reaganite Republican president preside over a civil war and lose power as a left-liberal takes over. Miller seems to sympathize with the left-liberal president but shows how completely ineffective he is.
I think he became a moron long before he became a conservative, though, and 9/11 just pointed him in a direction. I found Sin City and 300 unreadable self-parody, and his late-90s sequels to Give Me Liberty and Dark Knight were just dumb, dumb, dumb.
The irony of this is, in the Randian utopia Miller portrayed in GML, he depicted a cross between the General Assembly meetings at OWS and Rock-'Em-Sock-'Em Robots. I guess maybe if the Occupiers wore exo-skeletons and fought he'd be OK with it all?