A maritime union's decision to require its members to sign a loyalty oath to its constitution before they enter a local hiring hall may violate federal law, said a U.S. government official.

"This looks like a possible Hobbes Act violation," said an official with the Department of Labor Monday. The 1951 law prohibits groups involved in commerce from using economic threats or threats of violence against "any person or property."But an officer from the union responsible for the loyalty oath repudiated its use and said he would "immediately" order officials at the hiring hall to stop it.

"I was aware of the proposals to do this, but I had told our attorneys I did not agree or concur" with the statement, said Alexander C. Cullison, a vice president of District No. 1/Marine Engineers Beneficial Association/ National Maritime Union.

As reported previously, officers from District No. 1 are refusing to allow shipboard engineers to enter a hiring hall in Portland, Ore., unless they sign a statement promising to abide by the constitution and bylaws of MEBA/NMU.

The Labor Department official would not comment directly on MEBA/NMU, but said prohibiting members from entering a union hall because they would not sign the statement "looks like" a possible violation of the Hobbes Act.

"We have jurisdiction to look into it," he said. Violations of the act by labor unions are investigated by the Labor Department's Office of Racketeering or the Federal Bureau of Investigation.

The engineers last month voted to abolish their 1988 merger with the NMU and revert back to District No. 1-Pacific Coast District/MEBA, their status before the merger. Since that time, the District No. 1 president, C.E. ''Gene" DeFries, has moved to put their union under receivership.

In a sign of the confusion inside the union, lawyers and union officers with District No. 1 offered contradictory explanations of their actions.

Mr. Cullison said that a sign stating that the Portland hiring hall was for MEBA-NMU members was acceptable.

"But I see no utility or advantage in signing a loyalty oath," he said. ''I will find out if it's still being practiced and stop it."

"I'd be very interested in hearing this (Labor Department) man's theory,

because on the face of it, it's absurd," said Joe Kollick, an attorney with Dickstein, Shapiro and Morin, the Washington, D.C., law firm that represents District No. 1 /MEBA-NMU.

District No. 1, he said, is safeguarding its property in Portland and "not intimidating anyone. In essence, this is no different than someone at a Moose Lodge saying you can come in only if you're a member."

But Susan Souder, an attorney for the Ward group, said: "I think there's a world of difference between a private club and a union hall keeping members out. This oath says, if you're not in the DeFries union, you don't work. Economic intimidation is a very fearful thing for working people."

The Ward group - which has led the engineers since a 1990 election - controls the majority of MEBA's hiring halls, which dispatch engineers and captains to U.S.-flag shipping and tanker companies.

The only exceptions are small offices in Portland and Norfolk, Va., which are controlled by the DeFries group.

Shipping lines are seeking a court ruling to tell them which faction they should go to for employees. They are also seeking a decision to clarify outstanding issues involving joint employer-union committees, such as pension and health plans, said the steamship industry attorney.