Rexford Guy Tugwell, 88, one of the original members of President Franklin D. Roosevelt's "brains trust" and a principal adviser on New Deal programs, died Saturday at Cottage Hospital in Santa Barbara, Calif. He had cancer.

Mr Tugwell moved easily between the worlds of academe and high government policy. As a political and economic theorist, teacher, and administrator, he was guided by conviction that government planners must soften the hard edges of private economic competition and provide citizens a measure of social and economic security.

"The fluidity of change in society has always been the despair of theorists," Mr. Tugwell wrote in "The Industrial Displince," published in 1933. In an often-quoted passage in the same book, he said, "Liberals would like build the station while the trains are running; radicals prefer to blow up the station."

By the time "The Industrial Discipline" appeared, Mr. Tugwell, who described himself as a liberal, already was in Washington and serving as assistant secretary of agriculture. He had become a member of FDR's "brains trust" a year earlier when Raymond Moley, a fellow faculty member of Columbia University, persuaded him to join Roosevelt's campaign to move from the New York governor's mansion in Albany to the White House. The third member of the "trust" at the point was Adolf A. Berle, a law professorr at Columbia.

Mr Tugwell had joined Agriculture at the behest of Henry A. Wallace, the secretary. In 1934, he became under secretary of the department. In 1935 and 1936, he also ran the Rural Resettlement Administration.

As a believer in government economic planning, Mr. Tugwell found an ideal place in agriculture. In the early 1930s, 70 percent of Amercians still learned their living from the land. As the Great Depression deepened, they were the largest group affected. Their condition was thought to be notably susceptible to government planning.

Mr Tugwell supported programs to pay farmers to take land out of production rather than grow unneeded crops that would only have to be destroyed. He supported programs for soil conservation and rural electrification. He supported crop subsidies.

As head of the Rural Resettlement Administration, he was the overseer of programs to help families displaced from poor and unproductive farms relocate on good land. Th administration also provided help in introducing modern farming methods.

Mr Tugwell was one of the key organizers of the Civilian Conservation Corps, which provided jobs in forestry, land reclamation and similar projects for thousands of young men in the 1930s.

His influence within the first Roosevelt administration extended far beyond agriculture. He was a leading spokesman for many New Deal programs, including reform of the tax system, relief and public works.

In the housing field, he advocated the building of what became known as "Tugwell towns." The first of these was Greenbelt, Md., and it became a model for public and private efforts elsewhere in the country to provide low-cost housing in the suburbs.

In 1957, after 20 years away from the Washington scene, Mr. Tugwell returned to Greenbelt to live. He told an interviewer for the old Washington Daily News how he had got Greenbelt started.

"One day in the fall, 34 I asked the president if he'd go for a ride in the country," he said. "I brought him out here on what roads there were then and asked him waht he thought of it for a housing project. He fell in love with the place, so we got started right off.

"We were just getting started with the plans when Harry Hopkins just landed 3,500 workers here in trucks one morning-they were hungry men who needed work, so we put them to work and that, of course, made it expensive.

By the end of 1936, Mr. Tugwell had long been a lightning rod for conservative criticism of the New Deal. Partly for this reason, and partly because he was a liability for FDR's second presidential campaign, he resigned his government posts, although he remained close to the president.

After a brief period in private industry, he became chairman of the planning commission of New York City under Mayor Fiorello H. La-Guardia. In the post he drew up the city's first annual budget for capital improvements.

In 1940, Harold Ickes, then secretary of the Interior, asked Mr. Tugwell to go to Puerto Rico to devise ways to enforce a law prohibiting corporations from owning more than 500 acres if land on the island. There Mr. Tugwell met Luis Munoz Marin, then president of the Puerto Rican Senate and later governor of the island.

Munoz persuaded Mr. Tugwell to become chancellor of the University of Puerto Rico. The following year, President Roosevelt appointed Mr. Tugwell governor of the island. He held that post for five years.Among his accomplishments were the passage of a planning act and the establishment of a budget bureau, central statistical office and an economic development bank.

Mr. Tugwell later described his service in Puerto Rico as the second most productive period of his life.

In 1946, he became a professor of political science at the University of Chicago and also was director of the university's Institute of Planning. He retired in 1957 and moved to Greenbelt.

In 1959, Mr. Tugwell was the Hillman lecturer at Howard University.

In 1964, he joined the Center for the Study of Democratic Institutions in Santa Barbara. The institute had been started a year earlier by Robert M. Hutchins, a former chancellor of the University of Chicago. Mr. Tugwell took the post with the express understanding that he could devote his time to working out a proposal for a new constitution for the United States.

One version of it was published in 1970. If it were adopted, the states would be abolished and their place would be taken by 20 "republics," each of which would have at least 5 percent of the country's population. There would be six branches of government instead of the present three (the new branches would administer elections, set long-term development goals, and regualte and coordinate the work of all government administrative agencies). All political activities would be financed from tac revenues and private contributions would be banned.

Mr. Tugwell continued to wrok on his constitutional proposals until shortly before his death.

Mr. Tugwell was born at Sinclairville, N.Y., on July 10, 1891. His parents were Charles Henry and Dessie Redford Tugwell . His father was a prosperious cattle trader and canner. The young Tugwell grew up in Sinclairville and in Wilson, N.Y., where his father also had a cannery.

After attending high school in Buffalo, N.Y., he went to the Wharton School of Finance and Commerce at the University of Pennsylvania. There he earned bachelor's master's and doctoral degrees. After teaching briefly at the University of Wahsington and in Paris, he joined the economics faculty of Columbia University in 1920. He remained there until coming to Washington in 1933. He continued to hold his appointment at Columbia until he resigned in 1937.

Mr. Tugwell published or contributed to almost 30 books, including one in which he contributed a chapter on Soviet agriculture. This was based on a visit he made to the Soviet Union in 1927 with a group of trade unionists.

Other works concerned the threat of war in the nuclear age, and several books on President Roosevelt. The most recent, "Roosevelt's Revolution: The First Year-A Personal Perspective," appeared in 1977.

His book "The Brians Trust," published in 1967, won the Bancroft Prize in 1968.

Mr. Tugwell's marriage to the former Florence E. Arnold ended in divorce in 1938.

Later that year he married the former Grace Falke, who had been a student of his at Columbia and an assistant at the Agriculture Department. Survivors include his wife, of the home in Santa Barbara, and their two children, Tanis alexander, of Raleigh, N.C., and Marcia Tugwell, of Santa Barbara; two children by his first marriage, Army Maj. Tyler Tugwell, of Reston, and Dr. Franklin Tugwell, of Claremont, Calif; 11 grandchildren and three great-grandchildren. CAPTION: Picture, REXFORD G. TUGWELL