Look, McCain isn't going to win this election on domestic issues. NOBODY believes tax cuts are going to turn the economy around -- after all, the current situation occurred after seven years of tax cutting despite the additional (and far more important) massive help from the Fed in keeping interest rates low after the dot.com crash. Since neither Obama or Clinton has proposed reversing the Bush cuts for the middle class, for McCain to say, as he has, that "they're going to raise your taxes" doesn't compute unless you earn well into six figures, which less than 3-4% of the electorate does.
McCain can win in only three ways -- if he can convince the country that Obama/Clinton cannot defend it, or if the classic attack ads work as they did in 2004, or if the country simply refuses to elect a black or a female candidate. On the economy, he is speaking to no more than 10% of the electorate and there really isn't anything he can change in what he's said so far that doesn't totally paint him as a flip-flopper.
The only face in this year's election cycle that is MORE unable to do this is Romney, who called for a ZERO capital gains tax rate at one point. Add to that his already well-known flip-flopping on social issues and you've got a candidate whom Obama/Clinton's veep will chew up and spit out in the debates.
It says here that McCain chooses Charlie Crist, Governor of Florida, for two reasons: 1) McCain must win Florida at all costs and Crist is popular, and 2) although he's a tax cutter, he doesn't have a track record of being utterly senseless about it. And on other issues, he's a blank slate outside of Florida.
McCain probably doesn't need Crist to win Florida but he would pretty much lock up the state -- especially against Obama. The guy who WOULD HAVE BEEN nice to have around was George Allen of Virginia, who would have locked up what appears to be turning into a swing state. But Allen macaca-ed himself out of electability, and now it's the guy who beat him, Jim Webb, who may well end up running for vice president -- for the Democrats.