i buy the conventional wisdom that obama's best chance to get re-elected is the nomination of rick perry or michele bachmann. the best analogy would be to 1964 (in which the incumbent was elected). it's not that things were going so terribly well; it was (partly) the fact that goldwater was too radically conservative.
on the other hand, you don't know what might might happen should things get that far; say that we've slipped deep into a second recession by next summer and fall: unemployment rises to 10. then we'll all be praying with and for president perry. the perry presidency won't be as bad as you're expecting: it won't be a sheer theocracy. still it will be very very irritating.
