In the comments to an earlier thread, T Maysle provides this excellent description of how Clark undercut McCain's entire cultural claim to the White House:
The cultural assumptions behind it bear scrutiny. Its the mythic hero template, the trial by fire, the Christ-like suffering and near-death for a higher cause, for America, for you and me.
When the hero survives the unimaginable trial by fire, he assumes magical warrior chief status. He has passed the test of life, the test of the god or gods, and ascended to a higher plane above ordinary mortals and TV pundits.
A lot of McCain's selling point is based on this claim of assumed status. Clark is popping his balloon.
Clark isn't attacking McCain's service, he is attacking the cultural assumptions behind McCains' rationale to be President. No wonder the McCain campaign, and indeed the entire Republican apparatus, is freaking out. Talking about hitting a nerve.