Composition

Size of the court

Article III of the Constitution sets neither the size of the Supreme Court nor any specific positions on it (though the existence of the office of the chief justice is tacitly acknowledged in Article I, Section 3, Clause 6). Instead, these powers are entrusted to Congress, which initially established a six-member Supreme Court composed of a chief justice and five associate justices through the Judiciary Act of 1789. The size of the Court was first altered by an 1801 act which would have reduced the size of the court to five members upon its next vacancy, but an 1802 act promptly negated the 1801 act, legally restoring the court's size to six members before any such vacancy occurred. As the nation's boundaries grew, Congress added justices to correspond with the growing number of judicial circuits: seven in 1807, nine in 1837, and ten in 1863.

In 1866, at the behest of Chief Justice Chase and in an attempt to limit the power of Andrew Johnson, Congress passed an act providing that the next three justices to retire would not be replaced, which would thin the bench to seven justices by attrition. Consequently, one seat was removed in 1866 and a second in 1867. In 1869, however, the Circuit Judges Act returned the number of justices to nine, where it has since remained.

President Franklin D. Roosevelt attempted to expand the Court in 1937. His proposal envisioned the appointment of one additional justice for each incumbent justice who reached the age of 70 years 6 months and refused retirement, up to a maximum bench of 15 justices. The proposal was ostensibly to ease the burden of the docket on elderly judges, but the actual purpose was widely understood as an effort to "pack" the Court with justices who would support Roosevelt's New Deal. The plan, usually called the "court-packing plan", failed in Congress. Nevertheless, the Court's balance began to shift within months when Justice Willis Van Devanter retired and was replaced by Senator Hugo Black. By the end of 1941, Roosevelt had appointed seven associate justices and elevated Harlan F. Stone to Chief Justice.

Nomination, confirmation, and appointment

Article II, Section 2, Clause 2 of the United States Constitution, known as the Appointments Clause, empowers the president to nominate and, with the confirmation (advice and consent) of the United States Senate, to appoint public officials, including justices of the Supreme Court. This clause is one example of the system of checks and balances inherent in the Constitution. The president has the plenary power to nominate, while the Senate possesses the plenary power to reject or confirm the nominee. The Constitution sets no qualifications for service as a justice, thus a president may nominate anyone to serve, and the Senate may not set any qualifications or otherwise limit who the president can choose.

In modern times, the confirmation process has attracted considerable attention from the press and advocacy groups, which lobby senators to confirm or to reject a nominee depending on whether their track record aligns with the group's views. The Senate Judiciary Committee conducts hearings and votes on whether the nomination should go to the full Senate with a positive, negative or neutral report. The committee's practice of personally interviewing nominees is relatively recent. The first nominee to appear before the committee was Harlan Fiske Stone in 1925, who sought to quell concerns about his links to Wall Street, and the modern practice of questioning began with John Marshall Harlan II in 1955. Once the committee reports out the nomination, the full Senate considers it. Rejections are relatively uncommon; the Senate has explicitly rejected twelve Supreme Court nominees, most recently Robert Bork, nominated by President Ronald Reagan in 1987.

Although Senate rules do not necessarily allow a negative vote in committee to block a nomination, prior to 2017 a nomination could be blocked by filibuster once debate had begun in the full Senate. President Lyndon B. Johnson's nomination of sitting Associate Justice Abe Fortas to succeed Earl Warren as Chief Justice in 1968 was the first successful filibuster of a Supreme Court nominee. It included both Republican and Democratic senators concerned with Fortas's ethics. President Donald Trump's nomination of Neil Gorsuch to the seat left vacant by Antonin Scalia's death was the second. Unlike the Fortas filibuster, however, only Democratic Senators voted against cloture on the Gorsuch nomination, citing his perceived conservative judicial philosophy, and the Republican majority's prior refusal to take up President Barack Obama's nomination of Merrick Garland to fill the vacancy. This led the Republican majority to change the rules and eliminate the filibuster for Supreme Court nominations.

Not every Supreme Court nominee has received a floor vote in the Senate. A president may withdraw a nomination before an actual confirmation vote occurs, typically because it is clear that the Senate will reject the nominee; this occurred most recently with President George W. Bush's nomination of Harriet Miers in 2006. The Senate may also fail to act on a nomination, which expires at the end of the session. For example, President Dwight Eisenhower's first nomination of John Marshall Harlan II in November 1954 was not acted on by the Senate; Eisenhower re-nominated Harlan in January 1955, and Harlan was confirmed two months later. Most recently, as previously noted, the Senate failed to act on the March 2016 nomination of Merrick Garland; the nomination expired in January 2017, and the vacancy was filled by Neil Gorsuch, an appointee of President Trump.

Once the Senate confirms a nomination, the president must prepare and sign a commission, to which the Seal of the Department of Justice must be affixed, before the new justice can take office. The seniority of an associate justice is based on the commissioning date, not the confirmation or swearing-in date. The importance of commissioning is underscored by the case of Edwin M. Stanton. Although appointed to the court on December 19, 1869, by President Ulysses S. Grant and confirmed by the Senate a few days later, Stanton died on December 24, prior to receiving his commission. He is not, therefore, considered to have been an actual member of the court.

Before 1981, the approval process of justices was usually rapid. From the Truman through Nixon administrations, justices were typically approved within one month. From the Reagan administration to the present, however, the process has taken much longer. Some believe this is because Congress sees justices as playing a more political role than in the past. According to the Congressional Research Service, the average number of days from nomination to final Senate vote since 1975 is 67 days (2.2 months), while the median is 71 days (or 2.3 months).

Recess appointments

When the Senate is in recess, a president may make temporary appointments to fill vacancies. Recess appointees hold office only until the end of the next Senate session (less than two years). The Senate must confirm the nominee for them to continue serving; of the two chief justices and eleven associate justices who have received recess appointments, only Chief Justice John Rutledge was not subsequently confirmed.

No president since Dwight D. Eisenhower has made a recess appointment to the Court, and the practice has become rare and controversial even in lower federal courts. In 1960, after Eisenhower had made three such appointments, the Senate passed a "sense of the Senate" resolution that recess appointments to the Court should only be made in "unusual circumstances". Such resolutions are not legally binding but are an expression of Congress's views in the hope of guiding executive action.

The Supreme Court's 2014 decision in National Labor Relations Board v. Noel Canning limited the ability of the President to make recess appointments (including appointments to the Supreme Court); the Court ruled that the Senate decides when the Senate is in session (or in recess). Writing for the Court, Justice Breyer stated, "We hold that, for purposes of the Recess Appointments Clause, the Senate is in session when it says it is, provided that, under its own rules, it retains the capacity to transact Senate business." This ruling allows the Senate to prevent recess appointments through the use of pro-forma sessions.

Tenure

The Constitution provides that justices "shall hold their offices during good behavior" (unless appointed during a Senate recess). The term "good behavior" is understood to mean justices may serve for the remainder of their lives, unless they are impeached and convicted by Congress, resign, or retire. Only one justice has been impeached by the House of Representatives (Samuel Chase, March 1804), but he was acquitted in the Senate (March 1805). Moves to impeach sitting justices have occurred more recently (for example, William O. Douglas was the subject of hearings twice, in 1953 and again in 1970; and Abe Fortas resigned while hearings were being organized in 1969), but they did not reach a vote in the House. No mechanism exists for removing a justice who is permanently incapacitated by illness or injury, but unable (or unwilling) to resign.

Because justices have indefinite tenure, timing of vacancies can be unpredictable. Sometimes vacancies arise in quick succession, as in the early 1970s when Lewis F. Powell Jr. and William Rehnquist were nominated to replace Hugo Black and John Marshall Harlan II, who retired within a week of each other. Sometimes a great length of time passes between nominations, such as the eleven years between Stephen Breyer's nomination in 1994 to succeed Harry Blackmun and the nomination of John Roberts in 2005 to fill the seat of Sandra Day O'Connor (though Roberts' nomination was withdrawn and resubmitted for the role of chief justice after Rehnquist died).

Despite the variability, all but four presidents have been able to appoint at least one justice. William Henry Harrison died a month after taking office, though his successor (John Tyler) made an appointment during that presidential term. Likewise, Zachary Taylor died 16 months after taking office, but his successor (Millard Fillmore) also made a Supreme Court nomination before the end of that term. Andrew Johnson, who became president after the assassination of Abraham Lincoln, was denied the opportunity to appoint a justice by a reduction in the size of the court. Jimmy Carter is the only person elected president to have left office after at least one full term without having the opportunity to appoint a justice. Presidents James Monroe, Franklin D. Roosevelt, and George W. Bush each served a full term without an opportunity to appoint a justice, but made appointments during their subsequent terms in office. No president who has served more than one full term has gone without at least one opportunity to make an appointment.