There are two main things to remember about Mitt Romney. First, he's been on multiple sides of most every major issue. Second, he's currently on whatever side is most politically convenient.

Romney isn't some garden variety legislative flip-flopper who's changed his mind a few times or reversed support for a few pieces of legislation; he's a true political shape shifter whose core political principles and beliefs are almost impossible to determine.

The McCain campaign's 2007 "book" on Romney—a complete record of the McCain team's opposition research on their primary rival that Buzzfeed's Andrew Kaczynski leaked last night—doesn't contain much in the way of brand new information, but it certainly reinforces the picture of Romney as a person whose governing style goes whichever way the wind blows. In extensive detail, the book documents Romney's various turns on major issues—how he changed his mind on the Bush tax cuts, gun ownership, abortion, and immigration, just to pick a few issues. Romney's flip-floppery is so pervasive that it cannot be summed up except to say that it is a fundamental part of his political self.

But what the McCain book also demonstrates is how that translates into governance. Because Romney's positions aren't stable, his leadership on key issues isn't reliable. His boasts and explanations, meanwhile, are frequently slippery and evasive.

Romney repeatedly promised (and still claims) that he wouldn't raise taxes, but he raised more than $500 million in "fees" in 2003—including a fee on blind people (his proposed fee on the mentally retarded was apparently rejected)—more than any other U.S. state that year. Later he raised business taxes by more than $170 million under the guise of closing "corporate loopholes." On abortion, Romney ran as defiantly pro-choice in his losing 1994 Senate race, reaffirmed his pro-choice stance while running for governor, then switched to pro-life a few years after he got into office. He triumphantly claims he closed a $3 billion budget deficit after entering office, but that was a projected figure that never actually materialized. The actual deficit was closer to $1.7 billion. And when Romney left office in 2007, he handed the incoming Deval Patrick administration a $1.3 billion deficit.

And then there's spending. This year, Romney is running on a (not particularly credible) promise of serious federal spending reductions, and pointing to both his business record and his record as Massachusetts governor for evidence that he can make tough cuts. But as McCain's Romney book points out, state spending increased by 24 percent under Romney—that's $5 billion—in just three years, well above the rate of inflation. But by 2007, he was criticizing the federal government in Washington for spending far too much.

Romney's record and rhetoric since his first run for office suggests that the only constant over the years has been his flexibility. You may not know exactly which Romney is going to show up once in office, but it's a safe bet that it won't be the one people voted for.