The Supreme Court of the United States is the country's highest federal court. The Court has ultimate—and largely discretionary—appellate jurisdiction over all federal courts and state court cases involving issues of U.S. federal law, plus original jurisdiction over a small range of cases.
The nine Supreme Court justices base their decisions on their interpretation of both legal doctrine and the precedential application of laws in the past. In most cases, interpreting the law is relatively clear-cut and the justices decide unanimously; however, in more complicated or controversial cases, the Court is often divided.
In modern discourse, the justices of the Court are often categorized as having conservative, moderate, or liberal philosophies of law and of judicial interpretation. It has long been commonly assumed that justices' votes are a reflection of their judicial decision-making philosophy as well as their ideological leanings, personal attitudes, values, political philosophies, or policy preferences. A growing body of academic research has confirmed this understanding, as scholars have found that the justices largely vote in consonance with their perceived values. Analysts have used a variety of methods to deduce the specific perspective of each justice.
Partisan balance The simplest way to approximate the ideological leanings of Supreme Court justices is by the political party of the president who appointed them. In a 2000 paper, Segal, Timpone, and Howard found that, in their study area (civil liberties and economics cases from 1937 to 1994), presidents appear to be reasonably successful in extending their policy preferences by appointing like-minded justices to the court, though they found that justices appear to deviate over time away from the presidents who appointed them. In 1999, Pinello conducted a meta-analysis of 84 studies of American courts covering 222,789 cases adjudicated since World War II and found that political party affiliation was a dependable indicator of rulings: Democratic judges voted in favor of liberal solutions more often than Republican judges did, especially in federal courts (the U.S. Supreme Court, U.S. Courts of Appeal, and U.S. District Courts).
The graph below, based on data from the Supreme Court Database, shows this partisan split. In all the non-unanimous decisions made by the Supreme Court in the terms from 1937 to 2025 in which there was a specifiable ideological direction, justices appointed by Republican presidents (red bars) generally cast liberal votes much less frequently than did justices appointed by Democratic presidents (blue bars). At the top of this graph, the 7 justices, including two chief justices (marked "CJ") and 2 current justices (with solid red bars), who cast the lowest percentage of liberal votes were all appointed by Republican presidents. At the bottom of the graph, the 3 justices, including 1 current justice (with a solid blue bar), who cast the highest percentage of liberal votes were appointed by Democratic presidents. The current justices (those voting together since the 2023 term, shown with solid bars) are near the extremes for this period with all the 6 Republican-appointed justices near the top and the 3 Democratic-appointed justices near the bottom.
The graph below (using data from List of justices of the Supreme Court of the United States) shows the number of justices sitting in the Supreme Court who were appointed by Democratic or Republican presidents since 1936. In 1936, the Court had 7 justices appointed by Republican presidents and 2 appointed by Democratic presidents. Democratic president Franklin D. Roosevelt then filled the Supreme Court with 8 appointees in the late 1930s and 40s and promoted Chief Justice Harlan F. Stone, who had originally been appointed to the Court by Republican president Calvin Coolidge. Then Democratic president Harry S. Truman appointed 4 justices. In the 1950s, Republican president Dwight D. Eisenhower appointed 5 justices, reversing the partisan balance.
In the 1960s, Democratic presidents John F. Kennedy and Lyndon B. Johnson, appointed 2 justices each, flipping the balance back to a majority of Democratic-appointed justices. This was reversed by Republican president Richard Nixon who appointed 4 justices, followed by Republican presidents Gerald Ford, Ronald Reagan, and George H. W. Bush appointing a total of 7 more justices. (Democratic president Jimmy Carter was not able to appoint any justices in his single term). In succession, Democratic president Bill Clinton, Republican president George W. Bush, and then Democratic president Barack Obama each appointed 2 justices. Since 2020, with the appointment of 3 justices by Republican president Donald Trump, the Court has 6 justices appointed by Republican presidents. Democratic president Joe Biden appointed 1 justice, but that appointment did not change the partisan balance. As the graph clearly shows, in every term since 1970, the Court majority (consisting of at least 5 of the justices) has been appointed by Republican presidents. Every chief justice since 1953 has also been appointed by Republican presidents.
Devins and Baum point out that "before 2010, the Court never had clear ideological blocs that coincided with party lines". In choosing their appointments, presidents often focused more on friendship and political connections than on ideology. Republican presidents sometimes appointed liberals and Democratic presidents sometimes appointed conservatives. As a result, "... between 1790 and early 2010 there were only two decisions that the Guide to the U.S. Supreme Court designated as important and that had at least two dissenting votes in which the Justices divided along party lines, about one-half of one percent." Even in the turbulent 1960s and 1970s, Democratic and Republican elites tended to agree on some major issues, especially concerning civil rights and civil liberties—and so did the justices. But since 1991, ideology has been much more important in choosing justices—all Republican appointees have been committed conservatives and all Democratic appointees have been liberals. As the more moderate Republican justices retired, the court has become more partisan. The Court is now divided sharply along partisan lines with justices appointed by Republican presidents taking increasingly conservative positions and those appointed by Democrats taking moderate liberal positions. This alignment between the Justices' partisanship and their ideological leanings (known as "partisan sorting") appears to be greater than ever before.
A recent academic study indicates an earlier partisan split in one area: since 1920 justices appointed by Republican Presidents generally have made more pro-business decisions than those appointed by Democratic Presidents, and this pro-business propensity has grown since the 1950s and especially in the Roberts Court. Another study shows that, since the Warren Court in the 1950s, Republican-appointed justices have increasingly favored wealthy people and their interests.
Measuring ideological leanings To further discern the justices' ideological leanings, researchers have carefully analyzed the judicial rulings of the Supreme Court—the votes and written opinions of the justices—as well as their upbringing, their political party affiliation, their speeches, their political contributions before appointment, editorials written about them at the time of their Senate confirmation, the political climate in which they are appointed, confirmed, and work, and the political contributions of the law clerks they hire. Bonica and Sen provide an extensive overview of these methods.
From this data, scholars have inferred the ideological leanings of each justice and how the justices are likely to vote on upcoming cases. For example, Segal and Cover found a strong correlation (r=0.80) between justices' perceived ideological perspectives on civil liberties and civil rights issues as attributed to them in elite newspaper editorials written just before their confirmation (their Segal–Cover score) and their later votes in the study period 1953–1988. Epstein, Walker, and Dixon found they could explain and predict rulings in criminal justice cases (in the study period 1946–1986) using a simple model with four inputs: the political party affiliation of the majority of justices, the political party affiliation of the current president (representing the current political climate), the Supreme Court rulings in criminal justice cases in the previous year, and the percent of criminal cases the Court decides to hear in the current year (how much interest they take in the issue). In this analysis, the political party affiliation of the majority of justices provided about one-fourth of the predictive power. Bonica et al. utilized the ideology of the law clerks hired by federal judges (as estimated by the law clerks' political donations) to estimate the ideology of the judges themselves.
Using statistical analysis of Supreme Court votes, scholars found that an inferred value representing a justice's ideological preference on a simple conservative–liberal scale is sufficient to predict a large number of that justice's votes. For example, Grofman and Brazill performed multidimensional scaling (MDS) using SYSTAT 5.0 of the entire range of cases considered by the Supreme Court, 1953–1991. Analyzing terms with an unchanging membership ("natural courts") and a complete bench of nine members (3,363 cases), they found that a one-dimensional scale provided a satisfactory explanation of votes and that the degree of unidimensionality generally rose over the years, writing: "On average, over the 15 courts, the mean r2 values are 0.86 for a one dimensional metric MDS solution, and 0.97 for a two dimensional metric MDS solution." In 2003, Poole used various statistical measures to show that a unidimensional scale provided a good measure of the Rehnquist Court during the 8-year period 1995–2002.
Ideological leanings over time Using increasingly sophisticated statistical analysis, researchers have found that the policy preferences of many justices shift over time. The ideological leanings of justices and the drift over time can be seen clearly in the research results of two sets of scholars using somewhat different models:
Andrew D. Martin and Kevin M. Quinn have employed Markov chain Monte Carlo methods to fit a Bayesian statistic measurement model of ideal points (policy preferences on a one-dimensional scale) for all the justices based on the votes in every contested Supreme Court case since 1937. The graph below shows the results of their analysis: the ideological leaning of each justice from the term that began in October 1937 to the term that began in October 2022. Note that the scale and zero point are strongly dependent on the particular set of cases chosen to be reviewed by the Court each year; the relative distance between the lines for each justice is most informative. Each unique color represents a particular Supreme Court seat. The black lines represent the leanings of the chief justices. The yellow line represents the estimated location of the median justice—who, as Duncan Black's median voter theorem posits, is often the swing vote in closely divided decisions.
Michael A. Bailey used a slightly different Markov chain Monte Carlo Bayesian method to determine ideological leanings and made significantly different scaling assumptions.<ref name=Bailey2013> </ref><ref