U.S. District Court Judge Barrington D. Parker, in a ruling that could help avert a mail strike, yesterday rejected a bid by union dissidents to block a contract ratification vote by members of the nation's largest postal union.

Parker, dismissing dissidents' claims that a union advisory committee's rejection of the contract was final, said the planned referendum would serve the public's interest in uninterrupted mail service and postal workers' rights to the final word on their contract.

A notice of appeal was filed, but officials of the American Postal Workers Union, who had postponed the vote to await Parker's ruling, said they will begin mailing out ballots today to the union's nearly 300,000 members.

Three other postal unions, representing more than 200,000 other workers, are already in the midst of the contract ratification process, which is expected to take three weeks for all four unions to complete.

Although ratification is in some doubt in light of widespread attacks on the contract by union activists, top union officials have predicted that rank-and-file members - if given an opportunity to vote on the contract - would approve it.

There had been concern that an injunction barring the referendum might trigger strikes or other work interruptions, although they are technically barred by federal law.

An injunction probably would have meant submission of the APWU's contract to binding arbitration under provisions of U.S. postal laws. The Postal Service had indicated it would not return to the bargaining table.

The tentative contract negotiated July 21 between national officers of the four unions and the Postal Service retains an existing "no-layoffs" clause, which was the unions' principal demand, but provides relatively modest wage and cost-of-living gains: 19.5 percent over three years. This is considerably less than the 30 percent or more over three years that was recently won by unionized workers in the auto, steel, communications, coal and rail industries.

Judge Parker's order was the second assist from the federal bench that top union officials have received over the last few days in attempting to get the contract ratified without a strike. Earlier U.S. District Court Judge Frederick B. Lacey in New Jersey barred a strike vote by 23,000 New York-area APWU members that was aimed at triggering a nationwide walkout.

The contract's main problem now appears to come from within the unions themselves. Only a few days after the rejection vote by the APWU advisory committee last week, delegates to a convention of the National Association of Letter Carriers, the second largest postal union, called for the contract's rejection in a nonbinding voice vote. Rejection has also been urged by union leaders in several large cities, including New York.

The suit to bar the APWU ratification vote was filed by John Richards, president of the Pittsburgh-area local and a member of the national APWU bargaining advisory committee. He contended that the committee's 29-to-15 rejection of the contract was binding under the union's constitution, even though the committee later voted to submit the pact of the members fonaratification vote.

In his ruling, Judge Parker said the membership was the "ultimate authority" in all union matters and the committee could waive its veto power over the contract to promote the right-to-vote principle. To grant the injunction, he said, would amount to "virtual disenfranchisement" of nearly 300,000 union members by only a relatively few members of the advisory committee.

Moreover, he said, "the public interest in having the mail undisrupted is obvious" and conducting the referendum would serve this interest.

Richards' attorney, Joseph Pass, said he would seek an expedited appeal of the ruling and attempt to recover the estimated $250,000 cost of the vote from union officers if Richards is subsequently upheld.