The President pro tempore of the Senate is the second-highest-ranking official of the United States Senate and the highest-ranking senator. The Vice President of the United States is the President of the Senate ex officio, and thus is the highest-ranking official of the Senate; during his absence, the President pro tempore is the highest-ranking official in the Senate and may preside over its sessions. The President pro tempore is elected by the Senate; by custom, the President pro tempore is the most senior senator in the majority party. Normally, neither the Vice President of the United States nor the President pro tempore presides; instead, the duty is generally delegated to junior senators to help them learn parliamentary procedure. The President pro tempore is third in the line of succession to the Presidency, after the Vice President of the United States and the Speaker of the House of Representatives.
The office of President pro tempore was established in 1789 by the Constitution of the United States. Originally, the President pro tempore was appointed on a daily or weekly basis when the Vice President of the United States was not present to preside over the Senate. Until the 1960s, it was common practice for the Vice President to preside over daily Senate sessions, so the President pro tempore rarely presided over the Senate unless the Vice Presidency became vacant.
Until 1891, the President pro tempore only served until the return of the Vice President to the chair or the adjournment of a session of Congress. Between 1792 and 1886, the President pro tempore was second in the line of presidential succession following the Vice President and preceding the Speaker of the House of Representatives. Thus, when President Andrew Johnson, who had no Vice President, was impeached and tried in 1868, Senate President pro tempore Benjamin Franklin Wade was next in line to the Presidency. Wade's radicalism is thought by many historians to be a major reason why the Senate, which did not want to see Wade in the White House, acquitted Johnson. The President pro tempore and the Speaker were removed from the line of succession in 1886, but were restored in 1947. This time, however, the President pro tempore followed the Speaker of the House.
Following the resignation for health reasons of then-President pro tempore William P. Frye, a Congress divided among progressive Republicans, conservative Republicans, and Democrats reached a compromise by which each of their candidates would rotate holding the office from 1911 to 1913. (See Presidents pro tempore of the United States Senate, 1911-1913.)
The ceremonial post of Deputy President pro tempore was created for Hubert Humphrey, a former Vice President of the United States, in 1977 following his losing bid to become the Senate majority leader. The Senate resolution creating the position stated that any former President of the United States or former Vice President of the United States serving in the United States Senate would be entitled to this position. Since Humphrey's death in 1978, no other former President or former Vice President has served in the Senate. As of 2007, if they successfully sought election to the Senate, four living Presidents, (Jimmy Carter, George H.W. Bush, Bill Clinton, and George W. Bush), and four living Vice-Presidents, (Walter Mondale, Dan Quayle, Al Gore, and Dick Cheney), are eligible for the position of Deputy President pro tempore. Former Vice-President Walter Mondale's failed 2002 Senate election bid is the only time this appeared to be a real possibility.
When the President pro tempore becomes unable to perform the duties of office for an extended period, the current practice is to elect a Senator as Deputy President pro tempore, as opposed to a Permanent Acting President pro tempore, to carry out the duties until the President pro tempore can resume the duties. George J. Mitchell was elected Deputy President pro tempore in 1987–1988, because of the illness of President pro tempore John C. Stennis. The office to date has remained vacant. Hubert Humphrey and George J. Mitchell are the only Senators to date that have held the title.
The post may be purely honorary and ceremonial, but nevertheless, it comes with a salary. By statute, the compensation granted to the position holder equals the rate of annual compensation paid to the President pro tempore, Majority Leader, and Minority Leader. (See .)
President pro tempore emeritus is an honorary title given to a member of the minority party in the United States Senate who has previously served as President pro tempore. Thus, a new person gains the title only when party control of the Senate changes. Ted Stevens, a Republican from Alaska, has held the title since January 4, 2007. Stevens served as President pro tempore from 2003 to 2007.
The position was created in 2001 when the Democratic Party regained a majority in the Senate. With the change in party control, Democrat Robert Byrd of West Virginia was elected President pro tempore, reclaiming a position he had previously held from 1989 to 1995 and again briefly earlier in 2001. The President pro tempore who had held the office under Republican control, Strom Thurmond of South Carolina was elected President pro tempore emeritus. Thurmond served in that capacity from June 6, 2001 until his retirement from the Senate on January 3, 2003 which coincided with another party change (from Democratic to Republican control) making Byrd the second President pro tempore emeritus.
While the President pro tempore emeritus has no official duties, he is entitled to an increase in staff. The President pro tempore emeritus also works closely with party leaders and advises them on the functions of the Senate.
A President pro tempore emeritus whose party regains the majority can also serve again as President pro tempore, as happened at the beginning of the 110th Congress on January 4, 2007. When party control changed from Republican to Democratic, Robert Byrd reclaimed the position of President pro tempore from Ted Stevens, who became the third President pro tempore emeritus.