The past two years we have witnessed the transformation of John McCain from a maverick to a mimic. Still smarting from his 2000 primary loss to George W. Bush, and after soiling his reputation in 2004 by rushing to the president's reelection defense, by 2007 McCain realized that the only way he could make it to the White House was to change his stripes and adopt the reckless ideology and empty platitudes that brought Bush to power.
But let's be fair about McCain's historical voting pattern: If you look at McCain's National Journal voting history since he arrived in the Senate in 1987, he is a moderate. "In the first eight years after his 1986 election, McCain was typically among the more conservative GOP senators," writes the Journal. "But starting in 1995, he became more moderate." And his voting record most of this decade was pretty consistently down the middle. This was the old John McCain, the guy who once opposed Bush's tax cuts and supported immigration reform and campaign finance reform.
Of course, McCain missed so many votes in 2007 while running for president he didn't even earn a National Journal score last year. Once he set his sights on the White House, the former POW went MIA.
A related note: You might notice from the same link above that Barack Obama was ranked most liberal senator in 2007. This is the source of the Republican talking point you hear all the time. But Obama was not the most liberal in either 2005 (16th) or 2005 (10th). And while there is a strong case to make that Obama will in fact be the most liberal candidate ever elected to the presidency, should he win, if McCain trots out the "most liberal senator" line in the next two debates Obama ought to just say, "Well, at least I was around enough for the National Journal to give me a rating ... whereas you, Senator, have missed so many votes you don't have a rating." (On the other hand, I'm guessing Team Obama doesn't want to draw any more attention than necessary to that 2007 "most liberal" rating.)