WASHINGTON (AP) — Former U.S. Sen. Howard Baker Jr., who asked what President Richard Nixon knew about Watergate, has died. He was 88.

U.S. Sen. Lamar Alexander confirmed Baker's death in a news release Thursday.

Baker, a Republican, served 18 years in the Senate. He earned the respect of Republicans and Democrats alike and rose to the post of majority leader. He served as White House chief of staff at the end of the Reagan administration and was U.S. ambassador to Japan during President George W. Bush's first term.

His question at the 1973 Senate Watergate hearings — "What did the president know and when did he know it?" — became famous. It instantly focused the nation's attention on the cover-up that — more than the Watergate break-in itself — eventually brought down Nixon's presidency.