The key members of the Senate Labor Committee have urged Attorney General William French Smith to make sure the investigation of Labor Secretary Raymond J. Donovan includes all the unresolved allegations that have come to light since his appointment.

Committee Chairman Orrin G. Hatch (R-Utah) and Sen. Edward M. Kennedy (D-Mass.), the ranking minority member, contended in a joint letter that the charter of the special prosecutor appointed last week should be supplemented explicitly under the provisions of the Ethics in Government Act.

Only in this fashion, they suggested, could all the doubts concerning Donovan's activities as a New Jersey construction executive, before joining the Cabinet, be resolved effectively.

The letter was sent to the attorney general Dec. 30, the day after a special three-judge federal court named New York lawyer Leon Silverman to conduct a full-fledged investigation of an alleged $2,000 labor union payoff by an official of Donovan's company.

The judges also authorized Silverman to investigate any other alleged violation of federal criminal law on Donovan's part that might turn up during the inquiry into the asserted $2,000 payoff, but Hatch and Kennedy were evidently worried that this could be construed too narrowly.

They said they felt that the prosecutor's work should be "supplemented to include other unresolved matters," including Donovan's relationship with an alleged New York Mafia "soldier," William Masselli.

In all fairness to Donovan, the two senators said, the investigation also should cover any questions concerning Donovan's sworn testimony at last year's Senate confirmation hearings, as well as the "other allegations" made by the man who said he witnessed the $2,000 payoff.

That man, Mario Montuoro, former secretary-treasurer of Laborers Local 29 of New York, has charged that Donovan was present at a 1977 luncheon when another official of Donovan's company, Schiavone Construction, handed the president of Local 29 an envelope containing $2,000 in cash.

In addition, Montuoro and former Local 29 bookkeeper Joyce Cole have said that Local 29 officials reaped other benefits from Schiavone Construction, such as paychecks for "ghost employes" on New York City subway projects.

The Justice Department has yet to respond to the letter, which was distributed to other senators. A copy was obtained from official sources.

Silverman could not be reached for comment. Donovan's lawyer, Dean Burch, said Donovan had made it clear already that he would like all the charges coming from Montuoro and Cole "run down, no matter what they involve."

As for any other matters, Burch said, "I'm sure he Silverman will do what's needed to carry out a full and thorough investigation. I'm sure that's what the senators want, and that's what Ray Donovan wants."