Here's how it could happen:
At some point in mid August, John McCain will announce that he has decided that he can not accept his party's nomination for president. The reason will be health-related, and that may turn out to be the truth. Anyone who's seen him on stage these days knows he looks like he's about to keel over. And anyone who's been on a presidential campaign knows the physical demands are grueling and can be a challenge for a young man.
The reasons are simple. He can't win. Now that Obama is the presumptive Democratic nominee -- the polls all show that McCain's pro-war stance and Bush endorsement make him a lost cause in November. That combined with soft stand on litmus test conservative issues make him an unpopular candidate among the base. I know some Democrats that think the Republicans are planning to let McCain lose and 'sit this one out' so that they can hang the democrats with a bad economy and a war that is a morass. But that just isn't how they play. They play to win every hand -- think about 2000 with a popular Democratic president and good economy and a solid VP running for president. Why did they put up Bush? And why did they fight so hard? Because, you don't ever throw a game. And they're not going to throw this one.