Canada

That's the one.

I don't know if my memory of that era comports with actual history, but this is how I remember it playing out:

It looked like the Conservative attack ads were going to win the election for them again, just as they had against Dion and Ignatieff. They were ahead and gaining in the polls, and the Liberals seemed to have no response. The slogan was, "Trudeau: he's just not ready."

Then the polls stabilized for a few days, and the Liberals released that ad. The polls started rapidly reversing, and the Liberals decisively swept into power. I don't think I even saw another, "he's not ready," attack ad from the Conservatives again after that.

EDIT: One can debate how much of an effect that ad had, and whether Trudeau's actions matched it's promises, but for me it will always stick out as a good bit of political strategy.