The election of 1800 was decided in the House of Representatives due to a tie in the Electoral College between Thomas Jefferson and Aaron Burr. Both candidates, who were members of the Democratic-Republican Party, received 73 electoral votes, while the incumbent President John Adams, a Federalist, received 65 electoral votes.
At that time, the Constitution mandated that each elector cast two votes for President, with the candidate receiving the majority becoming President and the runner-up becoming Vice President. However, the tie between Jefferson and Burr created a situation where no candidate received a majority of the electoral votes.
As per the Twelfth Amendment, which later clarified the election process, the election was thrown to the House of Representatives to choose the President. Each state delegation in the House had one vote, and because the Federalists still held a majority in the House, there was initial reluctance to support Jefferson, who was a leading opponent of Federalist policies.
After several rounds of voting, and with some political maneuvering and negotiations, Jefferson ultimately secured enough support to be elected as President, while Burr became Vice President. This election highlighted weaknesses in the electoral process and led to the eventual passage of the Twelfth Amendment in 1804, which changed the way electors cast their votes, helping to prevent future electoral ties.