POUGHKEEPSIE, N.Y. -- Franklin Delano Roosevelt Jr., 74, the third son of President Franklin D. and Eleanor Roosevelt and a former U.S. representative, undersecretary of Commerce and director of the Office of Economic Opportunity, died of cancer Aug. 17 at a hospital here.

Mr. Roosevelt came of age while his father was in the White House. He began his political career in 1948, three years after his father's death. As the candidate of the Liberal Party, he defeated the Tammany Hall-backed Democratic candidate for New York's 20th Congressional District seat. He served three terms in the House, retiring in 1955.

When first elected to Congress, Mr. Roosevelt said his victory was "proof that we are experiencing a revolution in American politics" and predicted the end of "big-city party organizations formerly held by irresponsible clubhouse loafers."

Mr. Roosevelt lost the Democratic nomination for governor in 1954 to Averell Harriman, who was supported by Tammany boss Carmen Di Sapio. As a consolation, he was nominated for attorney general but lost that race to the popular Republican Jacob K. Javits.

During the race for the 1960 Democratic presidential nomination, Mr. Roosevelt was an early and enthusiastic supporter of Sen. John F. Kennedy (D-Mass.). This was of great importance to the Kennedy campaign because of what was perceived as the hostility of Eleanor Roosevelt to Kennedy.

Mr. Roosevelt devoted much time and energy to the cruicial West Virginia primary, where he helped transfer some of the enormous popularity of his late father to the junior senator from Massachusetts. Kennedy's victory in this primary over Sen. Hubert H. Humphrey was probably the most important event of the campaign for the Democratic presidential nomination.

After Kennedy's victory, Mr. Roosevelt was named undersecretary of commerce and was a frequent visitor at the White House. During the Johnson administration, he was named the first chairman of the Equal Employment Opportunity Commission. In 1966, he ran for New York governor on the Liberal ticket and was defeated.

At the time of his death, Mr. Roosevelt was chairman of the executive committee of the Mickelberry Corp., and chairman of the board of the Park Avenue Bank in New York. A former New York lawyer, he also had been a farmer and car distributor.

Mr. Roosevelt was born on Campobello Island, off the coast of Maine. It was on this island that his father was stricken with polio. Franklin Jr. was the fourth of five children. His sister, Anna Roosevelt Halstead, the eldest, died in 1975, and John, the youngest brother, died in 1981. Two of his brothers survive, James Roosevelt, who served six terms as a California representatives, and Elliott, a writer and rancher.

Mr. Roosevelt was a 1937 graduate of Harvard University and received his law degree from the University of Virginia in 1940. He served in the Navy during World War II. As a destroyer gunnery officer and executive officer, and later as skipper of the destroyer escort Ulvert M. Moore, he took part in the invasions of North Africa and Sicily, the Philippines, Iwo Jima and Okinawa. He also participated in the last two convoys to Murmansk. He was awarded the Silver Star, the Purple Heart and the Legion of Merit.

His marriages to Ethel duPont, Suzanne Perrin, Felicia Warburg Sarnoff and Patricia Oakes ended in divorce.

In addition to his brothers, survivors include his wife, the former Linda Stevenson Weicker, whom he married in 1984 and who lives at the home in Millbrook, N.Y.; five children by previous marriages, and eight grandchildren.