Tom Edsall helpfully recalls McCain's most famous attack ad--against George W. Bush in the 2000 South Carolina primary:
On February 1 that year, McCain emerged as the 19-point victor in the New Hampshire primary, well-positioned to put a dagger through George W. Bush's heart in the South Carolina primary - the contest Bush was banking on to stem his hemorrhage. Within days of losing New Hampshire to McCain, Bush nosedived from being a 20-point favorite in South Carolina to a 4-point underdog.
In one of their more artful tactical displays, Bush campaign allies accused McCain of fathering an illegitimate black child (McCain had adopted a Bangladeshi orphan) and of abandoning the cause of Vietnam vets missing in action.
McCain, who is known for his temper, took the Bush bait, becoming visibly enraged as he roamed the state and produced a television commercial in which he personally accused Bush of twisting "the truth like Clinton. We're all pretty tired of that....Do we really want another politician in the White House America can't trust?"
For one Republican to accuse another of being like Bill Clinton was, at that moment, beyond the pale.
"Suggesting that Governor Bush is as dishonest as Bill Clinton is a disservice to our party and our principles," Bush spokesman Ari Fleischer declared with all due righteousness. "Our nation has been through enough and John McCain's ad has gone too far."
I think Edsall may overstate the effect of the ad--he implies it was the deciding factor in the McCain flameout. But the broader point is certainly right: Negative ads require a certain amount of nuance and modulation, which McCain hasn't necessarily demonstrated when going negative in the past.