That would be my instinct, too. If the MSM portrays these events as "vicious attack dog" speeches, I would think that McCain would prefer not to have his VP tarnished right now. McCain would not want his VP's first actions to be defending what will be portrayed as "dirty" campaigning.
Not that I'm known as The Great Prognosticator, but...I'm picturing Romney and Giuliani as having fun with this. Attacking, to be sure, but smiley nonetheless. That said, I don't know why McCain would risk sacrificing his VP choice to this.
I’m thinking more that the anarchist-socialists gathering in the streets in Denver would be more likely to lynch one of the Republican candidates if they had the opportunity.
Best not to send any of the contenders for the ticket into the middle of a political riot.