Under Article I, Section 3, Clause 4 of the Constitution of the United States, the vice president of the United States is the ex officio president of the Senate but may only cast a vote in the Senate to break a tie. As of December 5, 2023, vice presidents have cast a total of 301 tie-breaking votes in the Senate.^[1] There are also two tie-breaking votes cast by Chief Justice Salmon P. Chase during the 1868 impeachment trial of President Andrew Johnson, although it is still debated whether he had the authority to do so.^[2]

Article I, Section 3, Clause 4 of the Constitution of the United States states:

The Vice President of the United States shall be President of the Senate, but shall have no Vote, unless they be equally divided.^[3]

John Adams, the first vice president of the United States, cast 29 tie-breaking votes during his tenure. His first vote was on July 18, 1789.^[4] He used his votes to preserve the president's sole authority over the removal of appointees,^[5] influence the location of the national capital,^[6] and prevent war with Great Britain.^[7] At times, he even convinced senators to vote against legislation that he opposed and often provided guidance on procedural and policy matters. Adams' political views and active role in the Senate made him a target for critics of the Washington administration. Toward the end of his first term, threatened by a resolution that would have silenced him on all but procedural and policy matters, he exercised more restraint, hoping to be elected president of the United States in his own right.^[4]

John C. Calhoun was the only vice president to cast tie-breaking votes against his own president, Andrew Jackson. In 1832, Calhoun cast a tie-breaking vote to delay and later defeat the nomination of Martin Van Buren as United States Minister to the United Kingdom. Calhoun's supporters in the Senate allowed him to defy Jackson, where just enough of Calhoun's faction abstained to create a tie that he then broke.^[8]

In the early 21st century, the threat of a filibuster increased the use of cloture to end debate in the Senate, especially on highly divisive issues,^[9][10] making it rare for the vice president to have the opportunity to cast a decisive vote, as cloture requires a three-fifths majority. However, rules changes in 2013 and 2017^[10] reduced the requirement for cloture on nominations to a simple majority, which led to the first use of a tie-breaking vote to confirm a Cabinet member when Vice President Mike Pence confirmed Betsy DeVos as secretary of education in 2017.^[11] In 2018, Pence cast a tie-breaking vote to confirm Jonathan A. Kobes to the Court of Appeals for the Eighth Circuit, the first such vote to confirm a judicial nominee in U.S. history.^[12]

Following the 2020 Senate elections, the Senate was divided 50-50 between Republicans and Democrats in the 117th Congress. Votes by Vice President Kamala Harris played a critical role in passing Democratic legislation, including the American Rescue Plan and the Inflation Reduction Act. In addition, Harris gave the Democratic Party majority-control of the Senate.^[13] On July 20, 2021, Harris broke Pence's record for the most tie-breaking votes in the first year of a vice presidency, casting seven tie-breaking votes in her first six months.^[14] She cast a total of 15 tie-breaking votes in her first year, setting a new record for the most tie-breaking votes in a single year in U.S. history, surpassing the 12 votes cast by John Adams in 1790.^[15] On May 11, 2022, Harris set a new record for tie-breaking votes in a single day, casting four votes.^[16] On July 13, 2023, Harris matched Calhoun's record for the most tie-breaking votes cast by a vice president in U.S history at 31 votes.^[17] On December 5, 2023, Harris broke Calhoun's record.^[18]

The recent dramatic rise in the frequency of vice presidential votes is due in part to the Senate often being equally divided on procedural votes, such as on invoking cloture, in addition to final votes on divisive issues, resulting in the vice president needing to vote multiple times on the same nominee or resolution.^[19]

As of December 5, 2023^[update], there have been 301 tie-breaking votes cast by vice presidents.^[15]