The presiding officer of the United States Senate is the person who presides over the United States Senate and is charged with maintaining order and decorum, recognizing members to speak, and interpreting the Senate's rules, practices, and precedents. Senate presiding officer is a role, not an actual office. The actual role is usually performed by one of three officials: the vice president of the United States; an elected United States senator; or, under certain circumstances, the chief justice of the United States. Outside the constitutionally mandated roles, the actual appointment of a person to do the job of presiding over the Senate as a body is governed by Rule I of the Standing Rules.
The United States Constitution establishes the vice president as president of the Senate, with the authority to cast a tie-breaking vote. Early vice presidents took an active role in regularly presiding over proceedings of the body, with the president pro tempore only being called on during the vice president's absence. During the 20th century, the role of the vice president evolved into more of an executive branch position. Now, the vice president is usually seen as an integral part of a president's administration and presides over the Senate only on ceremonial occasions or when a tie-breaking vote may be needed. It is now often used as one of the forms of checks and balances by the executive branch to the legislative.
The Constitution also provides for the selection of a president pro tempore of the Senate, to preside when the vice president is absent from the body (as the meaning of pro tempore, literally "for the time being"). The Constitution does not specify who can serve in this position, but the Senate has always elected one of its current members. By tradition, the position is given more-or-less automatically to the most senior senator of the majority party. In actual practice in the modern Senate, the president pro tempore also does not often serve in the role (though it is their constitutional right to do so). Instead, as governed by Rule I, they frequently designate a junior senator to preside.
When the Senate hears an impeachment trial of the incumbent president of the United States, by the procedure established in the Constitution, the chief justice presides.
Constitutional authority
The Constitution provides for two officers to preside over the Senate. Article One, Section 3, Clause 4 designates the vice president of the United States as the president of the Senate. In this capacity, the vice president was expected to preside at regular sessions of the Senate, casting votes only to break ties. From John Adams in 1789 to Richard Nixon in the 1950s, presiding over the Senate was the chief function of vice presidents, who had an office in the Capitol, received their staff support and office expenses through the legislative appropriations, and rarely were invited to participate in cabinet meetings or other executive activities. In 1961, Vice President Lyndon B. Johnson changed the vice presidency by moving his chief office from the Capitol to the White House, by directing his attention to executive functions, and by attending Senate sessions only at critical times when his vote, or ruling from the chair, might be necessary. Vice presidents since Johnson's time have followed his example.
Next, Article One, Section 3, Clause 5 provides that in the absence of the vice president the Senate could choose a president pro tempore to temporarily preside and perform the duties of the chair. Since vice presidents presided routinely in the 18th and 19th centuries, the Senate thought it necessary to choose a president pro tempore only for the limited periods when the vice president might be ill or otherwise absent. As a result, the Senate frequently elected several presidents pro tempore during a single session.
On three occasions during the 19th century, the Senate was without both a president and a president pro tempore: July 9–11, 1850, following Millard Fillmore's accession to the presidency upon the death of Zachary Taylor, until William R. King was elected president pro tempore; September 19–October 10, 1881, following Chester Arthur's accession to the presidency upon the death of James A. Garfield, until Thomas F. Bayard was elected president pro tempore; November 25–December 7, 1885, following the death of Vice President Thomas A. Hendricks, until John Sherman was elected president pro tempore.