Obama communications director Dan Pfeiffer tells Politico that McCain would be committing "political malpractice" by leaking his veep pick tonight. I disagree. McCain would be committing political malpractice by formally announcing his veep pick tonight, since Obama's speech would overshadow the announcement far more than vice versa. But leaking the pick seems win-win. It'll distract the media a bit, but most voters won't notice and will come to the news pretty fresh tomorrow.
Having said that, I think the potential gain here for McCain is exceedingly marginal. It's not like the networks are going to cut away from the Obama speech to report on a leak. It's not like the leak is going to bump Obama off page one tomorrow. At best, the Republican veep discussion creeps in slightly at the margins, but not in a way that really dampens the speech.