When I read it back then, I got the sense that Miller was writing with a certain characterization in mind. Granted, he's since revealed himself as a foaming, rightwing asshole, but at the time it was far less clear. Certainly his version of Reagan shows nothing of the reverence that one would expect from a standard Rightie. And professional curmudgeon Alan Moore, who more recently has castigated Miller for his idiotic Conservative views, wrote the forward to the original bound edition of Dark Knight Returns, wherein he commented on the political undertones but didn't take Miller to task for them at the time. It seems that Moore accepted these as a technic of character rather than as a piece of propaganda.
Miller also portrays Green Arrow, a character with famously left-leaning sensibilities, in an entertaining and favorable light, and in fact he provides a foil for Wayne's uncompromising all-or-nothing perspective.
It's also hard to pigeonhole 1986's Miller as a Rightwing writer when we consider that Batman: Year One came out at almost the same time, with a much different political tone, while Miller's Daredevil was far from a Conservative paragon. Likewise, Miller's horrendously bad Robocop II was an indictment of corporate culture in start contract to the pro-corporate mantras of typical Rightwing cultists.
I honestly believe that this is a case of Miller's later political views encouraging us to revisit his earlier work, but I'm not convinced he as as aggressively (or as shit-headedly) Rightwing back then as he later became.