To be honest, after the office of the Vice President, the average American citizen has no idea of the legal order of succession to fill a vacancy in the Office of President of the United States - or even where to find that order of succession? 
Under ordinary circumstances, the Constitution of the United States provides guidance for filling a vacated Presidency. Our Founding Fathers established the initial guidance for Removal, Death, Resignation, or Inability to discharge the Power and Duties of said Office in Article I, Section 1, Paragraph 6. Constitutionally, adjustments to the document with regard to the Presidential succession process were set in place, January 1933, with the ratification of the 20th Amendment; and again, in February 1967, with the ratification of the 25th Amendment. 
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