Well, usually when I say that stuff "rolls off their back like water off a duck" I mean that they're untouchable.
Okay, I read your post again.
Yes, I was saying that they're untouchable, and I stand by that. I take it now (after re-reading your post a couple times) that you're saying that "they know better than to think they're untouchable".
But they are untouchable, at least for the time being. They're not running attack ads because they think the Liberals, or the NDP, can touch them. They're running the ads to kill their opposition...sorry, define their opposition...before the opposition defines themselves.
As for the Liberals' experience, it didn't hit them like a brick until Sheila Fraser, whose reputation is basically gold, came out with an incredibly damning report. And a weaker leader in Martin (cp Chretien) responded by trying the crisis management tactic of "Owning" - taking a problem to heart in an effort to communicate that you're dealing with it.
That tactic failed in the face of strong evidence that they weren't, they hadn't, and they wouldn't. So Martin tried again with the Inquiry, which bought him some time, but ultimately, also failed to secure the public trust that Martin needed to counteract the growing anger.
The Conservatives' bluster and obfuscate approach to crisis management is at the opposite end of the spectrum as "Own" and works as long as their message resonates more than the message of those trying to rip them. And, in an environment where nobody's paying attention, it's working just fine for now.
I despise that it's working, but it's working.
There may well be an expiry date on how long the tactic works, but it's not yet obvious that such a date is coming in the imminent future. Until then, they won't (and, if you're advising them, shouldn't) change tactic.
As for what will eventually undermine their current approach, I think it will be economic conditions (as I outlined). I don't think enough people buy in to the idea that Canada's Ruling Party is as criminal and corrupt as is being purported in some circles to make Robocon stick.