Negative campaigning went too far. McCain was the victim of what many regard as the nadir of mudslinging in 2000.
Yes, that tactic by Karl Rove--push-polling in the South Carolina primary to suggest that McCain had "an illegitimate black child" (he and his wife had adopted a little girl from Banglidesh) was pretty slimy, but--"the nadir of mudslinging?" Please.
In the spring of 2000, Rove and the rest of the right-wing rumor mill were just getting warmed up.
Have we forgotten 2004, the year that added "swiftboating" and "flip-flop" to the political lexicon?
And the poisonous effects of the "illegitimate black child" smear didn't linger beyond the state primary for which it was engineered (and McCain has long since shown himself eager to forgive and forget on that one), as opposed to, say, the right-wing generated falsehood that Barack Obama is a Muslim, which is likely to pop up again and again for the rest of his political life.