The 25th Amendment to the U.S. Constitution outlines the order of authority should the president die, be impeached or resign from office. According to Section One, if the president is removed from office for any reason, the vice president steps in to serve as president.
Section Two states that if there is no vice president, the president nominates a new vice president who must be confirmed by both congressional houses.
Section Three continues that if the president informs the Senate or the House of Representatives that he is unable to act as president, power is transferred to the vice president.
Section Four adds that if the vice president and other high ranking officials inform Congress that the president is unable to perform his duties, the vice president assumes control unless the president petitions to Congress, which then must decide in favor of the president or the vice president.