The Dems just rejected their own version of McCainism when they ditched Dean. Why would they now want the Republican version of the same thing?
One difference though. McCain voted for, and was a vocal proponent and defender of, Bush's Iraq policy.
The top of our ticket is already blotted in the 'voted for' category. If there isn't balance at the bottom of the War ticket, Naderism, currently bubbling on low, could really boil over.