Stephen E. Smith, 62, who managed the 1980 presidential campaign of his brother-in-law, Sen. Edward M. Kennedy (D-Mass.), and managed the Kennedy family real estate and financial holdings, died Sunday at his home in New York. Family members declined to disclose the cause of death.

Known as a low-key political operative who shunned the spotlight, Mr. Smith, the husband of Jean Kennedy, was involved in Kennedy family campaigns since John F. Kennedy's 1958 Senate race.

In "The Making of the President 1960," Theodore H. White described a meeting at which Mr. Smith was "sitting quietly, unobserved as usual . . . . "

Smith, he noted, had come to Washington in January 1959 to open, "unannounced," the Kennedy campaign's first presidential headquarters.

According to White, the soft-spoken Smith rose rapidly in the organizational structure to become "unflustered administrative chief of all the impossibly complicated financial and personnel details of a campaign that eventually was to mobilize scores of thousands of volunteers and employees."

Subsequently, he played key roles in Edward Kennedy's first Senate campaign in 1962 and the 1964 campaign in which Robert F. Kennedy was elected to the Senate from New York. He also was a manager of Robert Kennedy's 1968 run for the Democratic presidential nomination.

Mentioned as a possible contender for New York governor in 1970, Mr. Smith probably played his most prominent political role in 1980, as manager of Edward Kennedy's unsuccessful campaign to wrest the Democratic presidential nomination from incumbent President Carter.

The grandson of a member of Congress, Mr. Smith was born in Bayport, N.Y., into a family prominent in harbor-related businesses. He worked on barges during summers, graduated from Georgetown University, served as an officer in the Air Force and worked in the family business.

In addition to his wife, survivors include four children, William, Stephen Jr., Amanda and Kym.