Supreme Court Justice Stevens has announced he will step down during Obama’s presidency to help ensure his replacement will maintain the current idealogical balance on the court. The Supreme Court has 9 justices, and most cases are decided by a 5-4 vote. In many cases, Justice Stevens represents the deciding 5th vote. Interestingly, Stevens was appointed by a Republican president, but he has tended to vote with the more liberal justices in recent history.
Stevens has been on the Court since 1975, and will turn 90 this month. He is second oldest justice in history, and is the seventh-longest-serving justice.