The Liberals know this as well as anyone, as seen during the last election with one of the most vicious attack ads ever aired on Canadian TV. In addition to going after Harper hard and shooting guns at viewers, it was far more misleading than anything the Tories have come out with so far this election.
And, for anyone needing a reminder, Stephen Taylor has posted a spoof of the ad on his site.
That said, it's a little odd to accuse your opponents of going negative because they have nothing else to offer...and then go negative yourself.
UPDATE: I guess one of the benefits of having such low production value is that you can churn out dozens of ads. Mike Duffy has just shown yet another Tory ad (not up on their website) with a picture of Paul and the words "you paid for it" being stamped down after listing various scandals.