The biggest existential threat to John McCain's presidential candidacy is?

Correct answer: Barack Obama

and now Ambinder post McCain spin the same way he post Hillary spin.

This doesn't quite capture McCain's problems. The war, yes; but also his betrayal of his moral high ground on torture, his 180 degree turn to embrace the worst elements of Bush/Rove politics, etc. In short, his problem is his loss of integrity and honesty.