Under the U.S. system of separation of powers, it is the president's job to choose Supreme Court nominees and the Senate's role to confirm or reject them.
After the president announces his choice, the appointment goes to the Senate Judiciary Committee, which conducts hearings on the nominee.The committee has the power to block the nomination, but given the importance of the Supreme Court nominees, it's likely the nomination will be forwarded to the full Senate.
Twenty-seven high court nominees in the nation's history have failed to win Senate confirmation, only seven in this century.
Most recently, the Senate in 1987 blocked President Ronald Reagan's appointment of Robert Bork to fill a vacancy created by the retirement of Justice Lewis F. Powell.
The Senate vote was 58-42, with six Republicans joining 52 Democrats in opposing Bork.
Reagan's second choice, Douglas Ginsburg, withdrew after revealing he had smoked marijuana. The Senate then confirmed Reagan's third pick, Anthony M. Kennedy.