There has never been a tie in a U.S. presidential election, but the Constitution outlines a process for resolving such a scenario.

Due to the way the Electoral College is structured, it is possible for a candidate to win an election despite losing the popular vote. This has happened only five times in U.S. history: in 1824 when John Quincy Adams defeated Andrew Jackson, in 1876 when Rutherford B. Hayes defeated Samuel Tilden, in 1888 when Grover Cleveland defeated Benjamin Harrison, in 2000 when George W. Bush defeated Al Gore, and in 2016 when Donald J. Trump defeated Hillary Clinton.

But if the 538 electors in the Electoral College split their vote at 269 to 269 and cannot agree on a candidate, then the House and Senate must step in to hold a contingent election. Here's what would happen and who would need to become involved if there were a tie in the Electoral College.

The U.S. Constitution

When the U.S. first gained its independence, Article II, Section 1 of the Constitution outlined the process for selecting electors and the process by which they would choose a president. At the time, electors could vote for two different candidates for president; whoever lost that vote would become vice president. This led to serious controversies in the elections of 1796 and 1800.

In response, Congress ratified the 12th Amendment in 1804. The amendment clarified the process by which electors should vote. More importantly, it described what to do in the event of an electoral tie. The amendment states that "the House of Representatives shall choose immediately, by ballot, the President” and “the Senate shall choose the Vice-President." The process is also used in the event that no candidate wins 270 or more Electoral College votes.

The House of Representatives

As directed by the 12th Amendment, the 435 members of the House of Representatives must make their first official duty the selection of the next president. Unlike the Electoral College system, where a larger population equals more votes, each of the 50 states in the House gets exactly one vote when selecting the president.

It is up to the delegation of representatives from each state to decide how their state will cast its one and only vote. Smaller states like Wyoming, Montana, and Vermont, with only one representative, wield as much power as California or New York. The District of Columbia does not get a vote in this process. The first candidate to win the votes of any 26 states is the new president. The 12th Amendment gives the House until the fourth day of March to select a president.

The Senate

At the same time that the House is selecting the new president, the Senate must select the new vice president. Each of the 100 senators gets one vote, with a simple majority of 51 senators required to select the vice president. Unlike the House, the 12th Amendment places no time limit on the Senate's selection of a vice president.

If There Is Still a Tie

With 50 votes in the House and 100 votes in the Senate, there could still be tie votes for both president and vice president. Under the 12th Amendment, as amended by the 20th Amendment, if the House has failed to select a new president by Jan. 20, the vice president-elect serves as acting president until the deadlock is resolved. In other words, the House keeps voting until the tie is broken.

This assumes that the Senate has selected a new vice president. If the Senate has failed to break a 50-50 tie for vice president, the Presidential Succession Act of 1947 specifies that the Speaker of the House will serve as acting president until tie votes in both the House and Senate have been broken.

What About Ties in a State’s Popular Vote

What would happen if a state’s popular presidential vote ever resulted in a tie? While statistically remote, tie votes are possible, particularly in smaller states. In the event a state’s popular vote was to result in an exact tie, a recount is required. Should the vote remain a tie even after the recount, state law governs how the tie is to be broken.

Similarly, an extremely close or disputed vote could result in a state run-off election or legal action to decide the winner. Under Federal law at 3 U.S.C. section 5, state law governs and would be conclusive in determining the state’s Electoral College vote. If the state has laws to determine controversies or contests as to the selection of its electors, the state must make that determination at least six days prior to the day the electors meet.

Past Election Controversies

In the controversial 1800 presidential election, an Electoral College tie vote occurred between Thomas Jefferson and his running mate, Aaron Burr. The tie-breaking vote made Jefferson president, with Burr declared vice president, as the Constitution required at the time. In 1824, none of the four candidates won the required majority vote in the Electoral College. The House elected John Quincy Adams president despite the fact that Andrew Jackson had won the popular vote and the most electoral votes.

In 1837, none of the vice-presidential candidates won a majority in the Electoral College. The Senate vote made Richard Mentor Johnson vice president over Francis Granger. Since then, there have been some very close calls. In 1876, Rutherford B. Hayes defeated Samuel Tilden by a single electoral vote, 185 to 184.﻿﻿ And in 2000, George W. Bush defeated Al Gore, 271 to 266 electoral votes in an election that ended in the Supreme Court.﻿﻿