From the Weekly Standard:

When it came time for questions, all anyone wanted to know was what is he doing in Boston? One reporter said Romney was "perturbed" that McCain was campaigning in Massachusetts today. McCain responded that he couldn't account for Romney's reaction, but that Romney was welcome to campaign in Arizona. When pressed on his bold prediction last week that he would seal the nomination on Tuesday, McCain backtracked-"I am not predicting that. I am not predicting that. I am guardedly confident that we can do well, but I am predicting nothing....we'll be campaigning all the way until the polls close tomorrow night, I don't think that's predicting victory." Well, yes, he is, but why in Massachusetts, which isn't a particularly tight race, instead of California, where some last minute stops might make a big difference?

One theory: McCain really doesn't like Romney, and as it became clear last week that McCain would almost certainly secure the nomination on Tuesday, this was to be a parting slap in the face. Except today, while McCain is still a heavy favorite, the result in California looks far from certain and there's a real possibility this race will continue past tomorrow's contest. It's hard to see how this was a smart move, but it was pure McCain--inspiring, audacious, and a bit reckless.