US National Archives
Nara: Presidential Election Laws: Presidential Election Laws
Check out this site from the National Archives and Records Administration on the US Constitution and the United States Code for laws and provisions regarding presidential elections.
Annenberg Foundation
Annenberg Classroom: Twentieth Amendment
Annenberg Classroom material with text of the 20th Amendment along with an explanation of its meaning. Links to timeline and video resources.
Annenberg Foundation
Annenberg Classroom: Terms and Succession
Check out this interactive timeline that documents the history of terms of succession in the United States.