I'll agree with that to a degree. The Conservatives' attack ads actually turned me right off from them. It still didn't influence whom I'd voted for, but it certainly helped me to decide whom not to vote for.
If any party tries that again this year, it will probably influence me in the same way. in other words, if you want my vote, don't tell me why I shouldn't vote for the other guy, don't tell me the other guy is a worst candidate than you are. Your best strategy if you want my vote is to ignore the other guy and just show me why I should vote for you. Heck, you can even feel free to poach good ideas from the other guy, acknowledge that he has some good ideas, and that might even increase my likelihood of voting for you since it would show that you recognize a good idea when you see one, even when it comes from an opponent.
When you waste your time attacking others, that tells me that that's all you'll be doing once in power too. If you focus on promoting good policies, that is a good sign.