Among the most effective campaign ad strategy is to use your opponents own words against them. The McCain campaign uses that strategy by using Biden's words against Obama. Biden's "Obama will be tested by the world if elected" comment (Berman Post: Biden - Obama Will be Tested by The World if Elected). The ad ends saying that it does not have to happen, meaning that the United States will not be tested if McCain is elected. Its hard to beat audio like this, and the visuals are good serving to emphasis the message. I think it will be pretty effective.
Video embedded below.