Obama's speech on national security was not accidentally scheduled on the same day as Cheney. In politics who you choose to fight with is as important as what you say. Why not choose another republican. Why not RNC chair Michael Steele? Or Palin or any of the others?
Its because by choosing the opponent you also choose how you shape your message. Obama chose an old time GOP guy who everyone knows and only 25% of the population likes. By making war with Cheney, Obama goes after the image of the old white guy who would drown anyone who gets in his way. Obama's message is juxtaposed against Cheney's. On one hand you have a constituational scholar saying that torture actually hurts U.S. interests, helps recruit terrorists, and reduces our stature in the world. On the other hand you have a cranky old figure head saying waterboarding actually helped the U.S.
The same happened back in 2004. During the dem primary fight between Dean, Kerry, and Edwards, Bush had started attacking Kerry which elevated Kerry position in the dem party and eventually giving Kerry the dem nomination. At the end of the day, why did Bush go after Kerry? Look at the opponent Bush chose: New England, rich, private school, entitled senator. The person they were worried about was Edwards. Southern, came from nothing, good looking. (But who really knew what a douche Edwards ended up being.)
