Jeane J. Kirkpatrick, a conservative political scientist who became the first woman to serve as U.S. ambassador to the United Nations, died at her home outside Washington late Thursday, colleagues announced today. She was 80.

A notice of her death was posted this morning on the Web site of the American Enterprise Institute, a conservative think tank in Washington where Kirkpatrick worked as a senior fellow.

"The United States has lost a great patriot and a champion of freedom, and AEI mourns our beloved colleague," the announcement said.

Kirkpatrick "defended the cause of freedom at a pivotal time in world history," and her "powerful intellect helped America win the Cold War," President Bush said in a statement.

"As a professor, author, ambassador and adviser to presidents, she influenced the thinking of generations of Americans on the importance of American leadership in advancing the cause of freedom and democracy around the globe," Bush said. "Her insights and teachings will continue to illuminate the path ahead for the United States in the world."

Kirkpatrick's assistant, Andrea Harrington, said she died in her sleep late Thursday at her home in Bethesda, just north of Washington, D.C., in the Maryland suburbs, the Associated Press reported. The cause of death was not immediately known, but Harrington said Kirkpatrick had been in declining health recently and was "basically confined to her house."

In New York, John R. Bolton, the outgoing U.S. ambassador to the United Nations, asked for a moment of silence for Kirkpatrick at his morning staff meeting.

Senate Majority Leader Bill Frist (R-Tenn.) said in a statement that Kirkpatrick "stood up for the interests of America while at the U.N., lent a powerful moral voice to Reagan foreign policy, and has been a source of wise counsel to our nation since leaving government service two decades ago. She will be greatly missed."

Born Jeane Duane Jordan in Duncan, Okla., in November 1926, Kirkpatrick graduated from Barnard College in 1948 and received a doctorate in political science from Columbia University in 1968. She also studied political science in Paris.

She was a leftist early in her academic career and later joined the Democratic Party, becoming active in party politics and political campaigns in the 1970s. But she grew disillusioned with the foreign policy of President Jimmy Carter and eventually left the party, aligning herself with the conservative policies of Ronald Reagan.

She joined the American Enterprise Institute as a senior fellow in 1978. She also spent long stretches of her career as a professor of government at Georgetown University.

Kirkpatrick advised Reagan on foreign policy during his 1980 presidential campaign, and he nominated her as U.S. ambassador to the United Nations after defeating Carter that year. Kirkpatrick served in the U.N. post from 1981 to 1985 and was a member of Reagan's Cabinet.

After leaving the administration, she returned to the American Enterprise Institute and to Georgetown University.

At the United Nations, "she never forgot who she was representing," Bolton told reporters this morning in a brief tribute to Kirkpatrick. "She was a great scholar. She was one of the most outstanding advocates of American foreign policy in our history."

Bolton, who formerly worked with Kirkpatrick at AEI, became emotional as he recalled her influence on him. "I benefited very greatly," he said, his voice quaking.

"She spoke clearly for liberty in the world [and] made it clear during tensions in the Cold War that America's interests here at the U.N. were advanced when the cause of liberty was advanced," Bolton said.

During her time at the United Nations, Kirkpatrick became known as an outspoken anti-communist and a staunch defender of Israel.

She continued to speak out on public issues and to remain involved in foreign and defense policy matters long after leaving the Reagan administration.

She served on the President's Foreign Intelligence Advisory Board from 1985 to 1990 and on the Defense Policy Review Board from 1985 to 1993.

In 2002, she warned at a pro-Israel seminar that establishment of a Palestinian state would be "a catastrophic mistake" that would endanger Israel and appease terrorists.

Three years later, she joined seven other former U.N. ambassadors in cautioning U.S. lawmakers against a plan to withhold dues to prod the United Nations to reform itself. Such a move, they said in a letter to Congress, was misguided and would "create resentment, build animosity and actually strengthen opponents of reform."

Although Kirkpatrick once described herself as a lifelong Democrat, she formally became a Republican in 1985. She considered seeking the Republican presidential nomination that went to George H.W. Bush in 1988, AP reported, but retreated to the position that she would accept the vice presidential nomination if asked.

Reflecting at a 2002 conference on her early career as a socialist, she said it had been "relatively short." As she read the works of various socialists, she said, "I came to the conclusion that almost all of them, including my grandfather, were engaged in an effort to change human nature. The more I thought about it, the more I thought this was not likely to be a successful effort. So I turned my attention more and more to political philosophy and less and less to socialist activism of any kind."

Kirkpatrick is survived by two sons, Stuart Alan Kirkpatrick, a Buddhist minister in Ann Arbor, Mich., and John Evron Kirkpatrick, a lawyer in Miami. A third son, Douglas Jordan Kirkpatrick, died earlier this year. Her husband of 40 years, Evron Maurice Kirkpatrick, a scholar and former member of the World War II-era Office of Strategic Services, died in 1995.