The Constitution’s separation of powers among the executive, legislative, and judicial branches of government is based largely upon John Adams’s essay "Thoughts on Government," published in 1776. Rather than centralizing power in a single branch, which could become tyrannical, each branch uses a system of checks and balances to limit the extent of the others. The Supreme Court is the highest court in the judicial branch, and its power rests on its ability to influence the executive and legislative branches through judicial review of constitutionality. Laws and executive orders can be invalidated by the Supreme Court if they are deemed unconstitutional.
Supreme Court justices hold their offices during good behavior, which means they retain their position for life unless they are impeached. Impeachment has has only happened once, in 1804. While this is meant to protect the justices’ decisions from the partisan influence of the election cycle, it cannot entirely remove the role of politics from the judicial branch. Justices are seated following a partisan process, meaning a nomination by the president and appointment by the Senate. Their prior opinions, usually as federal court judges, are taken into consideration in order to choose a potential Supreme Court justice who is likely to side with the political party in power Supreme Court justices serve lifetime appointments, which means that their own political sympathies, as indicated through their record of liberal or conservative interpretation, can influence the laws of the United States for generations.
The executive branch chooses Supreme Court nominees, which are then appointed or rejected by the legislative branch. Legislative checks upon the judiciary are not limited to approving or disapproving nominees; Congress is also able to decide how many justices serve on the Supreme Court. The number of justices has changed multiple times: the Judiciary Act of 1789 established a Supreme Court with six justices; the Judiciary Act of 1801 limited the court to five; FDR tried to expand the number to fifteen in 1937, but failed to do so; and the number has remained nine since the Judiciary Act of 1869, leaving the number of justices odd to prevent ties. Additionally, the legislative branch can amend the Constitution to overturn judicial branch rulings of unconstitutionality, and it can impeach Supreme Court justices if they are convicted of treason, bribery, or other high crimes.