Nate Silver says dropping Palin doesn't make electoral sense for McCain:

Not only would this hurt McCain, but it would also harm downballot candidates; the odds of Democrats finishing with 60+ Senate seats or 260+ House seats would increase markedly.

If he drops her or she quits, it's devastating. Why? Because it's all about his judgment and executive skills. He did no vetting and made his pick in an instant of insanity. This was his first serious presidential-level decision. It makes Bush's decision-making look Solomonic. If McCain is forced to acknowledge this, his own campaign is over too. So they cannot give in; they have to double-down; they will train her to do something crazy and polarizing in the debate. They will pray that Biden is sexist or condescending and, given Biden, that may not be too big a gamble. There will be more fireworks and more gambits and more nuttiness: just to prevent Americans from thinking through the real decision before them.

But Americans will still have to ask themselves: could we trust Palin as leader of the free world at a moment's notice? And: why did McCain present us with this option? Unlike the pundits, the voters have to check reality. And the Palin reality is objectively horrifying.