As I said in my original "endorsement," McCain has some big-government tendencies I dislike. I also think he's by far the best bet for limiting government among any of the people who were or are running this year.
How was the big leftward shift after 2000 supposed to be politically advantageous? I mean, say you're John McCain, and you just got crushed in SC et al for not being conservative enough, and you want to run for president again. It seems to me that either you deliberately plan to switch over to the Dems, and then actually do it, or you put your head down and spend the next 4-8 years becoming the most reliable conservative in the Senate. What you don't do is tack wildly to the left for the first couple years, thereby pissing off everyone in your party, but simultaneously refuse to actually make the switch to Dem, thereby preventing your new positions from doing you any real good; and then spend the next several years tacking wildly back to the right. As you observe, it's his positions right after 2000 that are causing him so much grief now. If he's a coldly cunning strategist, he's not actually very good at it. Besides which, it's not just that he does things which tick off the party, and might benefit him politically; he also does things which tick off locals, and don't seem to benefit him at all. What was the grand thought behind opposing the disaster-fund-whatever-thingy in FL? He dang near lost FL. What was the thought behind telling the Michiganners they weren't getting their jobs back? He did lose MI. If you ask me, Matt Yglesias' characterisation of McCain as 'flighty', or Matt Welch's discussion of McCain's quasi-religious hyper-nationalism (Welch has a great interview on bloggingheads) are both more convincing than the calculating Iceman hypothesis.
UPDATE: And yes, I do include in the above "any of the people" the ridiculous Ron Paul, whose spending plans were only 5 percent lower than McCain's, and whose views on immigration and social issues had little to do with limiting government.