I Am Tomorrow — Lester Del Rey

Thomas Blake has just become president. He has plans to change the world. To be a benevolent leader that will give the power to the people. But when he is wrenched out of his body and finds himself in the body of a rebel assassin who's job is to kill the Bigshot dictator. The dictator is Blake forty years hence, and now he was obviously falling in love with a girl who believed his highest mission in life was to shoot his older self!