In a precedent-setting move, all 19 staff attorneys of a specialized Washington law firm have organized themselves into a union and negotiated a contract with the firm's partners setting their wages, hours and fringe benefits.

Although small groups of salaried lawyers working for Legal Service Corp. projects, the government and public defender offices began organizing themselves into unions about five years ago, this is the first known example of lawyers in private practice joining a trade union.

The firm that has become unionized is Donald M. Murtha & Associates, a partnership established to handle one massive case -- the claim of 80,000 postal workers that they are owed more than $100 million in back pay for attending training and study sessions required by the U.S. Postal Service.

These suits can drag on for as long as 10 years, but the firm will probably disband when all the cases are closed. As a result, the large number of attorneys hired to handle this litigation -- 19 are already on salary, and another 21 soon may join them -- have little chance to be promoted to partner and share the profits the way lawyers in most firms do.

"While we are clearly in a temporary project with no hope of becoming partners, in practice more and more attorneys are going to find themselves in similiar positions," said Howard Kallem, one of the organizers of the union.

"We may be breaking ground by making it easier down the line for attorneys in more traditional private firms to organize," he said.

The main partner in Murtha and Associates, which is setting up offices near 15th and H streets, is Donald M. Murtha, an old-time labor lawyer who helped write and implement the nation's wage and hour laws during the 1930s, when he was a Labor Department attorney.

The other main partners are Gerald Fedder, former chief counsel to a Senate labor subcommittee who helped write the 1974 amendments to the Fair Labor Standards Act that is the basis of the postal workers' suit, and Jules Bernstein, associate general counsel for the Laborers International Union of North America.

They will share the bulk of the legal fees, expected to be in the millions of dollars. Staff lawyers will receive annual salaries ranging from $16,000 to $31,350 -- wages far below those paid by top Washington law firms, where a starting lawyer can earn as much as $27,000 a year.

Secretaries and paralegals in major New York law firms have begun organizing drives, but lawyers -- with the promise of partnerships and six-figure incomes -- have stayed away from the union movement.

An American Bar Association official, who said he is unaware of any other lawyers in a private law firm joining the union, added, "There's just no incentive for lawyers other than those in legal services or the public sector to organize."

Nonetheless, some of the young lawyers with the Murtha firm are considering hooking their independent union to a major international union so they can organize other lawyers in private practice. Those who work for legal clinics or as in-house attorneys for major corporations -- both growing areas in the field of law -- are considered prime targets for union organizing.

Negotiations with the Murtha firm started in December and the contract was signed a week ago yesterday. While the partners -- all labor lawyers who generally are on the union side of an issue -- quickly agreed to recognize the attorneys as a bargaining unit, they just as quickly switched hats to conduct tough negotiating session as bosses.

"They were very willing and ready to accept us as a union," said Doris S. Freedman, one of the union organizers. "As soon as we sought recognition it was given. But as soon as it was given, they accepted the role of management."

"They didn't give the store away in any way, shape or form," added Tim James, another Murtha attorney.

Like most law firms, Murtha Associates had not defined such common fringe benefits as vacation times or hours employes should work. In negotiating the contract, the unionized lawyers said, they aimed to make sure they were not being exploited by management while avoiding a time clock mentality that they feared would cut their professionalism.

The contract contains one clause rare to union documents which asserts that the "overriding responsibility" of both the partners and the staff attorneys is to make sure the clients get the best possible representation.

Moreover, the contract recognizes the higher authority of the lawyers' code of ethics and states that "all provisions or understandings covered by this agreement are subordinate to the prevailing moral and ethical considerations involved in the practice of law."