Look up the "United States presidential line of succession", there is an entire order for who becomes President in case the acting president is no longer able to carry out the duties of their office, are killed/presumed dead, or abdicate their position. While succession has been planned out to the point of dozens of successors, the top five in order are:

Vice President
Speaker of the House of Representatives
President pro tempore of the Senate
Secretary of State
Secretary of the Treasury

If you pay attention to major US political events, you will notice that you will never see all of those people in the same place at any given time; even during the State of the Union in Congress.