If no candidate for president receives an absolute majority of the electoral votes (since 1964, 270 of the 538 electoral votes), then the Twelfth Amendment requires the House of Representatives to go into session immediately to choose a president. In this event, the House of Representatives is limited to choosing from among the three candidates who received the most electoral votes for president. Each state delegation votes en bloc—each delegation having a single vote; the District of Columbia does not get to vote. A candidate must receive an absolute majority of state delegation votes (i.e., from 1959 (which is the last time a new state was admitted to the union), a minimum of 26 votes) in order for that candidate to become the president-elect. Additionally, delegations from at least two thirds of all the states must be present for voting to take place. The House continues balloting until it elects a president.

The House of Representatives has been required to choose the president only twice: in 1801 under Article II, Section 1, Clause 3; and in 1825 under the Twelfth Amendment.