Robert R. Carr, 48, a former executive director of the Fund for Constitutional Government who was active in drug law reform, social advocacy groups and Democratic politics, died of cancer Saturday at his Washington home.

Mr. Carr served as executive director of the fund one of the publishers of the Almanac of American Politics, in 1979 and 1980.

Before that, he served for five years as senior program officer and then senior consultant for the Drug Abuse Council, a private foundation engaged in public education and policy research into non-medical drug use in this country. He was a major contributor to the council's final report, "The Facts About Drug Abuse," published in 1980. The council went out of business that year.

In 1978, he served on a presidential commission task panel on psychoactive drug use and misuse.

Mr. Carr had worked as a political campaign director in Vermont before moving to Washington in 1967 as a legislative assistant to Donald Rumsfeld, then a Republican congressman from Illinois. He served as an administrative assistant to Rep. John Conyers Jr. (D-Mich.) from 1971 to 1973. He also had served as an information director for Eugene McCarthy's 1968 presidential campaign, as a platform committee liaison man for the Democratic National Committee in 1976, and, until his illness, as a member of the staff of the 1980 Democratic National Convention.

In addition, he had worked for Planned Parenthood and the Day Care and Child Development Council of America.

A native of Chicago, Mr. Carr earned bachelor's and master's degrees from Bradley University in Peoria, Ill., where he was voted an outstanding debater. He did postgraduate work at the University of Illinois.

From 1956 to 1966, he taught communications and coached debating teams at several colleges and universities.

He was a longtime member of the Americans for Democratic Action and served as chairman of the Greater Washington Chapter in 1969 and 1970. He had served on the national board of the ADA since 1976. He also was a member of the American Civil Liberties Union.

Survivors include his wife, Judith, of Washington; two daughters, Colette and Michele, both of Madison, Wis.; a brother, Dennis, of Tucson, Ariz., and a sister, Joan Lovejoy of Tempe, Ariz.

The family suggests that expressions of sympathy be in the form of contributions to the St. Francis Center, Washington, D.C.