A middle-ranking supervisor of welfare investigations in the D.C. Department of Human Resources complained at a Senate hearing yesterday that his bosses have stripped his office of vital personnel-and he got immediate action.

The whistle blower who won a victory was John B. Stewart, who has been chief of DHR's office of eligibility review ever since the unit was created by Congress in 1972 to investigate reports of widespread welfare abuse, including illegal overpayments.

Hearing Stewart's testimony that the unit lacks 18 of its authorized 45 investigators and administrators because many have been assigned other departmental duties, Sen. Patrick J. Leahy (D-Vt.) bluntly told DHR director Albert P. Russo to restore them to duty immediately.

Leahy, chairman of the Senate D.C. appropriations subcommittee, which passes on the city budget, delivered a blistering lecture to Russo.

"Does anybody at city hall listen to what we say? . . . I've voted for these investigators every damn year since I've been here, and they haven't even been used," Leahy said, pounding the dais with his fist. "The credibility of the city sinks to a new low."

Russo acknowledged that the 18 workers were in other important jobs. "I am not contesting Mr. Stewart's statistics," Russo told Leahy, "but I am pleading for fairness."

Stewart said that since June 1975 his staff "has been used so frequently on special projects and details that it has been difficult to plan regularly assigned duties and impossible to maintain any kind of production standards."

Even with reduced personnel, Stewart said, four investigators managed since 1975 to recover $456,936 from people illegally claiming welfare at the same time that they were drawing unemployment compensation.

Stewart said he carried his complaint to the Senate after trying to work within the department hierarchy. He said all 27 current employes of his office signed a petition seeking action.

In all, yesterday was a bad day for Russo, who already was smarting under a public reprimand delivered last week by Mayor Marion Barry because of DHR's failure to enforce a new law that provides for the licensing of group homes. The reprimand followed a fire April 10 that killed 10 women in a home for St. Elizabeths Hospital outpatients.

Leahy, hearing Russo's statement earlier in the day's testimony that the incidence of welfare payment errors was reduced 8 percent in the latest reporting period, said DHR must move faster or face threats of a congressional cutback of the city budget.

Russo said the department has filled most of the 187 jobs authorized by Congress last year to review and certify the eligibility twice a year of the 30,000 families on the city's biggest welfare program, which aids impoverished families with children. The program costs $85 million a year.

"Very soon, we are going to see a dramatic reduction [in errors]," Russo said. "If we don't see it, heads are going to roll, including possibly my own."