He flip-flopped as a fiscal conservative:
"Part of McCain's motivation was political. According to Wicke, he and his aides figured that these subsidies could attract several pro-nuclear Republicans, and they had their eyes on Senator Lindsey Graham of South Carolina and Senator Liddy Dole of North Carolina. Wicke was concerned at the time that the nuclear subsidies would cost the measure support and that
a bill loaded with money for the nuclear energy industry would contradict McCain's high-profile opposition to subsidies—which was partly responsible for his reputation as a fiscal conservative and a maverick. In June 2003, McCain had joined 47 other senators to vote for an amendment stripping an energy bill of up to $16 billion in subsidies for the nuclear power industry. (The amendment lost by a two-vote margin.)"
http://www.motherjones.com/washington_dispatch/2008/03/john-mccain-nuclear-waste.html