WASHINGTON - Conservative activist Paul Weyrich, 66, who coined the phrase "moral majority" and helped turn social conservatives into a powerful force in the Republican Party, died yesterday.

Mr. Weyrich's death was announced by the Heritage Foundation, a conservative think thank that he had helped to create. No cause was given. Friends said that he suffered from ill health and that both legs had been amputated due to diabetes.

Republican National Committee Chairman Mike Duncan said Mr. Weyrich "was instrumental in the development of conservative thought" in America. Weyrich was the first president of the Heritage Foundation and the leader of other conservative organizations, and his service "embodied and further advanced the Republican Party's core values of limited government, lower taxes and individual responsibility," Duncan said.

Mr. Weyrich, who lived in northern Virginia, was one of three founders of the Moral Majority, and later had a hand in creating the Christian Coalition. "He was a dedicated conservative and patriot, an excellent strategist," said Lee Edwards of the Heritage Foundation. "He had a very sharp sense of humor, which he employed at all times." At his death, Mr. Weyrich was chairman and chief executive officer of the Free Congress Foundation, a conservative think tank.

His latest commentary, posted yesterday on the foundation's Web site, said: "It is the worst of times because conservatives appear lost and without a serious agenda." But "it is the best of times" because conservative thinkers are generating ideas for a "Next Conservatism."

Mr. Weyrich got his start as a reporter in Milwaukee, and came to Washington in 1967 as press secretary to Sen. Gordon Allott (R., Colo.). Six years later, he founded the Heritage Foundation, and the next year the Free Congress Foundation. At a 1979 gathering of religious leaders, he talked of a "moral majority" in the country. The name stuck. Over the next decade, the group led by the Rev. Jerry Falwell energized the conservative movement as a political force.