 Because the Constitution reads that if the president cannot fulfill his or her duties, then a vice president would step into the role. You could not allow for the person, the president, it's believed that you couldn't allow for the president to turn around and be vice president because by Constitution, the vice president, if something should happen to the president, has to step in and become president. And the Constitution prevents anybody who served twice two full terms as president from ever serving again. Now, what was that third question?