Adam Nagourney makes the case:

Consider this: Perhaps the single most politically devastating moment for Mr. Deeds was when he gave a halting and fumbling answer, in a cluster of reporters and television cameras, about whether he would raise taxes to pay for repairing the state’s transportation system. Republicans have used clips from it to produce two of the most devastating advertisements of the campaign, raising questions at once about his views on taxes and his basic candor.

Mr. McAuliffe was given to his excess. But years of doing television interviews, on forums ranging from “Meet the Press” to scrums of local reporters — first defending the Clintons and than advancing himself and his party — paid off in producing a candidate who was an assured television presence.

That's true, but it certainly doesn't mean Republicans would have been unable to come up with potentially even more devastating ads against McAuliffe. Nagourney mentions how the summer arrest of the McAuliffe ally and fundraiser Hassan Nemazee on fraud charges "would not have been a happy moment in a McAuliffe campaign"; but I think it probably would have been a lot worse than that. In fact, it's easy to see how, after a steady barrage of GOP attack ads, Virginia voters might have believed that Nemazee was McAuliffe's running mate.