Palin boosted McCain’s numbers by probably 15%, maybe more. I distinctly remember him getting an almost immediate 8-10 point bump.
She is the only thing that kept him from getting blown out. Had he selected Lieberman, he would have lost by 25-30 points.
Of course, McCain’s ego won’t ever let him admit that.
If McCain had chosen Lieberman, it would have been a repeat of 1936 or 1964 with disastrous results for the GOP.