Another thing to keep in mind is that in both 1972 and 1988, the Republican incumbent presidents were much more popular than Bush. In July 1988, Reagan‘s approval rating was at 54 percent, while in June of 1972, Nixon‘s was at 57 percent. As I write, Bush is at 32 percent.
It’s only fair to note, per Stacy’s point, that McCain’s own favorable rating is still at a strong 62 percent, so there’s certainly an opening for him if Obama comes to be seen as too liberal. I just wonder how McCain gets more of those people who seem to have a favorable view of him actually vote for him, especially given the Republican fatigue.