The 71-year-old Democrat, with his trademark white shock of hair and slow professorial speech, is serving his fourth six-year term and would have been expected to be re-elected easily in 2000. He is a former chairman of the Senate Finance Committee.
The aides who revealed Moynihan's decision spoke on condition of anonymity.
Word of Moynihan's retirement came just three days after his longtime New York colleague, Republican Sen. Alfonse D'Amato, was defeated in his bid for a fourth term.
For years, the two men complemented each other with Moynihan focusing on foreign affairs, social policy and other broad issues and D'Amato concentrating on bringing home projects to his state.
Moynihan is the first senator facing re-election in 2000 to announce his retirement. With Republicans failing to expand their 55-45 Senate majority in Tuesday's voting, Democrats have been hoping to recapture control of the chamber that year, when 19 Republican and 14 Democratic seats will be at stake.
Moynihan is the top Democrat on the Senate Finance Committee, which oversees taxes, Social Security, Medicare, Medicaid and welfare all of which are at the heart of the Capitol's annual political fights.
He was the panel's chairman when Democrats held the Senate majority in 1993 and 1994. During those years and afterwards, he has been a maverick who at times has clashed with President Clinton.
He questioned Clinton's decision to focus early in his presidency on revamping the nation's health care system rather than fixing welfare, and in 1996 he criticized Clinton for signing the Republican-written welfare overhaul bill.
Moynihan grew up the rough lower Manhattan neighborhood known as Hell's Kitchen. His father was an alcoholic who left the family when Moynihan was 6. His mother operated a saloon.
Born in Tulsa, Okla., Moynihan became an academic who taught at Harvard before going into public service.
One of the first times he captured public attention was in 1965, when his report, The Negro Family, warned that the growth of single-parent families was only worsening inner cities. He wrote that "benign neglect" would be better than continuing failed welfare programs an argument that led to criticism by some that he was racist.
Moynihan served in the Kennedy, Johnson, Nixon and Ford administrations. Nixon named Moynihan ambassador to India in 1973. Ford made him United Nations ambassador in 1975. Known for bluntly asserting U.S. positions, Moynihan stayed at the United Nations only eight months before returning to Harvard.
In 1976, he won a contentious four-way Democratic Senate primary that included feminist Bella Abzug. He went o to defeat the conservative Republican incumbent, James Buckley. He was re-elected easily three times after that.
In the Senate, he is known for lofty oratory that often included the history of whatever program or institution is being debated.
And while he is usually a reliable Democratic vote, he can sometimes stray.
In 1993, he opposed a treaty lowering trade hurdles in North America that Clinton was pushing.
And in the early 1990s, he proposed cutting Social Security taxes to reduce the huge surpluses that system has been building up for future retirees, arguing that the money was merely being used to mask the period's large budget deficits. That proposal was rejected twice by the Senate.
For more than a year there has been speculation that Moynihan would not run again even though he had said he planned to do so.
Interest among Democrats in the Moynihan seat has been keen. On Thursday, newly re-elected state Comptroller H. Carl McCall, the only black elected to statewide office in New York, said he might be interested in the seat and refused to rule out a primary against Moynihan.
Also thought to be interested in the seat are U.S. Housing Secretary Andrew Cuomo, son of former New York Gov. Mario Cuomo; and Robert F. Kennedy Jr., son of the assassinated New York senator. The younger Cuomo is married to the younger Kennedy's sister, Kerry.
On the Republican side, New York City Mayor Rudolph Giuliani and U.S. Rep. Rick Lazio of Long Island are considered potential candidates. There has even been some speculation that D'Amato might be coaxed into running.
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