If the the President cannot serve then the line of authority goes down to the Vice President
After that, the line of authority goes down this way:
Speaker of the House of Representatives
President Pro Tempore of the Senate
Secretary of State
Secretary of the Treasury
Secretary of Defense
Attorney General
Secretary of the Interior
Secretary of Agriculture
Secretary of Commerce
Secretary of Labor
Secretary of Health and Human Services
Secretary of Housing and Urban Development
Secretary of Transportation
Secretary of Energy
Secretary of Education
Secretary of Veterans Affairs
Secretary of Homeland Security (not yet set by law)
01 August 2003