Kennedy's sudden illness elicited an outpouring of sympathy from Republicans and Democrats alike, including all three of the remaining candidates for president.

Sen. Barack Obama, D-Ill., whom Kennedy endorsed as the standard-bearer of President John F. Kennedy's legacy, said he spoke with the senator's wife.

"I have been in contact with the family. Obviously, they are in our thoughts and prayers," Obama said.

Sen. Hillary Rodham Clinton, D-N.Y., echoed the sentiment, as did Sen. John McCain, R-Ariz., the presumptive Republican nominee.

Kennedy was elected to the Senate in 1962, just two years after his brother, John Kennedy, was elected president and a year before his assassination.

He emerged as one of the Senate's most prolific legislators and successful dealmakers. He teamed with President Bush during Bush's first term to co-author the president's signature No Child Left Behind education legislation but broke dramatically with Bush over the invasion of Iraq and has been one of Bush's fiercest critics ever since.

His loss to then-President Jimmy Carter for the 1980 Democratic presidential nomination cemented his reputation as the champion of the party's liberal wing. Last August, he became only the third senator to have cast 15,000 votes.