Federal marshals may be assigned to protect Secretary of Labor Raymond J. Donovan's chief accuser, Mario Montuoro, in light of a reported gangland threat on his life, it was learned yesterday.

Montuoro, sources said, asked for the protection at a recent meeting with Special Prosecutor Leon Silverman, but Montuoro later had some misgivings he has yet to resolve.

Formerly secretary-treasurer of what government prosecutors have described as a Mafia-dominated labor union local, Montuoro, it was said, had been told that there had been a New York mob "meeting" or "sitdown" concerning him last week and that "they decided to kill him."

The report apparently came from someone of Montuoro's acquaintance, but under circumstances that Silverman evidently regarded as credible.

Meanwhile, other sources said, Attorney General William French Smith has brushed aside a senatorial request that he take steps to broaden the investigation of alleged union corruption involving Donovan.

Smith took the position that he has "no control" over the scope of the inquiry that Silverman has undertaken since his appointment last month by a special three-judge federal court.

The Ethics in Government Act, however, explicitly provides that the special court "may expand" the prosecutor's jurisdiction "upon request of the attorney general."

There was no immediate explanation from the Justice Department about the apparent discrepancy.

A federal grand jury in New York is expected to begin hearing evidence next week into the one charge that Silverman was clearly commissioned to investigate: Donovan's alleged presence in mid-1977 at a $2,000 payoff of a New York labor union official by another executive of Donovan's construction company.

But Silverman has repeatedly refused to comment on whether he intends to look into other unresolved allegations about Donovan and his firm, the Schiavone Construction Co., that have come to light since his nomination in December, 1980, as secretary of labor.

The special prosecutor was authorized by the court in its Dec. 29 order "to investigate any other allegation or evidence of violation of any federal criminal law by Secretary Donovan developed during" the inquiry into Montuoro's account of the $2,000 cash payment. But Silverman has given what appear to be conflicting signals to different individuals who have spoken with him about how broadly, or narrowly, he interprets that authority.

Concerned that it might be construed too narrowly, Senate Labor Committee Chairman Orrin G. Hatch (R-Utah) and Sen. Edward M. Kennedy (D-Mass.), the committee's ranking minority member, urged Smith in a Dec. 30 letter to make sure the investigation covers all the unresolved allegations.

The two senators asked that Silverman's charter be explicitly "supplemented" to include, for example, the connections between Schiavone Construction and a subcontractor headed by an alleged Mafia "soldier" as well as Montuoro's "other allegations" about other favors Schiavone Construction assertedly provided officials of Montuoro's union, Laborers Local 29.

There is no indication thus far of any inquiry by Silverman into these matters.

Smith's response to Hatch and Kennedy took cognizance of their "belief that the mandate of the special prosecutor should be broadened."

The attorney general declared, however, that the prosecutor's mandate "was fashioned by the court in its order of Dec. 29, 1981, and is a matter over which the Department of Justice has no control."

Montuoro has been outspoken about the alleged corruption in Local 29 since he was ousted from his posts there in 1978. He has said he first told federal authorities that year of a luncheon at a Long Island City restaurant at which a Schiavone official handed an envelope containing $2,000 in cash to Local 29 President Louis Sanzo.

According to Montuoro, Donovan, then executive vice president of Schiavone and still (through a trustee) one of its principal owners, was present at the luncheon table. The labor secretary has denied it and called Montuoro a liar.

Montuoro and his lawyer, Arthur Schwartz of New York, have repeatedly urged a thorough probe of all the allegations involving Schiavone Construction. So has Donovan himself. Schwartz has been especially vocal on the issue because, he says, the five-year statute of limitations will come into play in a few months and automatically bar prosecution of most of Montuoro's allegations.