The Clinton camp must love the fact that McCain is in the lead.
They know how much he is disliked.
McCain is like one of those gazelles you see in a nature show crossing the river when Hillary the crocodile is waiting beneath the surface.
Obviously there are those who voted for McCain, but soon a small fraction will be willing to admit it.
I think it was Winston Churchill who said something to the effect that democracy seemed like a good idea until one spent a few minutes talking with the "average" voter.
Plus, McCain's nomination will take experience - neither has been governor or mayor - and immigration off the table as issue. Both of them voted against the Cornyn amendment.
Also, McCain is a short guy, thus she will not appear as short and dumpy as she would next to Romney - unfortunately some people do vote on physical appearance. She will look attractive compared to weird looking McCain whose face will remind people of a steroid user's. He has lots of baggage and has recently been a more outrageous liar than she. It will also make Bill's past irrelevant since McCain will be bringing Cindy - not somebody many would approve as First Lady. In contrast, Hillary will be the faithful wife while he is the adulterer.