if the base isn't strongly on board because of who is at the top of the ticket - and who the opponent is, then its a base not worth having.
If Bush loses this election, and low turnout amongst the base is the reason, the Republican party is going to shift left. Because having a base that can't provide 50.1% of the vote, and stops you from trying to get additional votes that can, isn't worth having - it's political suicide.
No, you're wrong, there are some who criticize Bush from the right, but are willing to give him another term, rather than sit out the election. Any move leftward, including a Rino VP, would be an indication of weakness of commitment to conservative values and result in a loss of votes.