It definitely would lessen my opinion of Thompson. And it definitely would not change my decision not to vote for McCain if McCain gets the nomination. My dream would be that this thing goes to a brokered convention, McCain is not picked and gets so pissed off that he retires from politics or at least leaves the Republican party. I despise the man.
Your dream could quickly become a nightmare if McCain spitefully decides to run as a third-party candidate (possibly with Michael Bloomberg's money since McCain would have no problem naming him VP). His third-party candidancy would ruin any chance the Republican nominee has in the general election.