Federal mediators intervened yesterday in contract talks between the U.S Postal Service and two unions representing most of the service's organized workers in an attempt to get an agreement before the current national pact expires at midnight July 20.

Kenneth E. Moffett, acting director of the Federal Mediation and Conciliation Service, and Nick Fidandis, director of mediation, held a joint session yesterday with postal officials and leaders of the American Postal Workers Union and the National Association of Letter Carriers, which represent 500,000 of nearly 600,000 unionized postal employes.

Fidandis said the meeting was a "formal intervention" by the mediation service in the month-long talks, which have been characterized by public bickering and mutual changes of bad-faith bargaining.

"We've intervened on our own because of the facts of the situation, because of the critical nature of this thing," Fidandis said. He said the mediation service has authority to intervene in postal labor talks under the 1970 Postal Reorganization Act, which granted postal workers collective bargaining rights.

Union sources confirmed yesterday that the so-called "main table" talks, in which the two largest unions are bargaining jointly, are going badly.

About 1,000 APWU and NALC representatives from around the country came to Washington yesterday to hear their leaders discuss those concerns in a closed-door "emergency meeting."

Postal officials yesterday declined comment on the "main table" talks, but said negotiations were going smoothly with the two smaller unions -- the Mail Handlers and the National Rural Letter Carriers Association.