CRS report via FAS – Contingent Election of the President and Vice President by Congress: Perspectives and Contemporary Analysis, Thomas H. Neale, Specialist in American National Government, November 3, 2016.

The 12th Amendment to the Constitution requires that presidential and vice presidential candidates gain “a majority of the whole number of Electors appointed” in order to win election. With a total of 538 electors representing the 50 states and the District of Columbia, 270 electoral votes is the “magic number,” the arithmetic majority necessary to win the presidency. What would happen if no candidate won a majority of electoral votes? In these circumstances, the 12th Amendment also provides that the House of Representatives would elect the President, and the Senate would elect the Vice President, in a procedure known as “contingent election.” Contingent election has been implemented twice in the nation’s history under the 12th Amendment: first, to elect the President in 1825, and second, the Vice President in 1837. In a contingent election, the House would choose among the three candidates who received the most electoral votes. Each state, regardless of population, casts a single vote for President in a contingent election. Representatives of states with two or more Representatives would therefore need to conduct an internal poll within their state delegation to decide which candidate would receive the state’s single vote. A majority of state votes, 26 or more, is required to elect, and the House must vote “immediately” and “by ballot.” Additional precedents exist from 1825, but they would not be binding on the House in a contemporary election. In a contingent election, the Senate elects the Vice President, choosing one of the two candidates who received the most electoral votes.Each Senator casts a single vote, and the votes ofa majority of the whole Senate, 51 or more, are necessary to elect. The District of Columbia, which is not a state, would not participate in a contingent election, despite the fact that it casts three electoral votes.”