US Chief Justice William Rehnquist, a conservative force on the Supreme Court for over 30 years, has died after battling thyroid cancer since October, a court spokeswoman said.
Justice Rehnquist, 80, experienced "a precipitous decline in his health in the last couple of days" and died in the evening at his home in Arlington, Virginia, surrounded by his three children, court spokeswoman Kathy Arberg said.
As chief justice, Rehnquist pushed the closely divided nine-member court to the right, and President George W. Bush was expected to use the opening to continue his own drive to add conservative voices to the judiciary.
The Supreme Court has enormous power at the top of the judicial branch of the US government. Its decisions on constitutional issues are final.
Justice Rehnquist's death came just days before the Senate was to begin confirmation hearings for John Roberts, a conservative appeals court judge picked by Mr Bush in July to replace the more moderate Justice Sandra Day O'Connor and whose choice ignited a partisan clash. The court's new term opens on October 3rd.
Justice Rehnquist carved out a record as one of the most conservative jurists in US history on a court closely divided on such contentious issues as abortion, the death penalty and separation of church and state.
He was named by President Richard Nixon to the court and served for nearly 15 years before being elevated by President Ronald Reagan in 1986 to be the 16th chief justice.
Rehnquist voted with the conservative majority to expand states' rights in several areas and allow more public funding of religious activities.
He dissented on some landmark decisions on social issues, including the 1973 ruling that women have a constitutional right to an abortion and from the 2003 rulings upholding gay rights and the use of race in student admissions at public universities.
As the nation's top judicial officer, Rehnquist presided over President Bill Clinton's historic impeachment trial before the Senate in early 1999.
Rehnquist joined the court's conservative majority as a bitterly divided Supreme Court ruled by a 5-4 vote in 2000 to stop ballot recounts in Florida, effectively giving the presidency to Republican George W. Bush over Democrat Al Gore.