The nation's largest federal union filed suit for back overtime pay on behalf of several thousand government workers yesterday, three days after rival unions announced a similar lawsuit.

The rush to court is the latest skirmish in a representation battle between the federal sector's largest union, the American Federation of Government Employees (AFGE) and its closest rival, the National Treasury Employees Union (NTEU).

The AFGE has long represented 60,000 workers at the Social Security Administration, and the Treasury Employees Union voted this week to try to take away this important element of AFGE's strength.

One former federal labor official said the treasury union has made its reputation by being fast on its feet into the courthouse and is using the technique as an organizing tactic.

The NTEU filed suit July 19 on behalf of employes of the Social Security Administration and was joined in its filing in the northern district of Georgia by the third-largest federal union, the National Federation of Federal Employees. About 20,000 of AFGE's approximately 180,000 active members work for Social Security.

The AFGE rushed into U.S. Claims Court here yesterday, seeking back overtime "pay and damages for GS-11 workers who would have received overtime pay if the Office of Personnel Management had honored the Fair Labor Standards Act." The act requires overtime pay for all employes who do not hold executive, administrative or professional prositions.

Robert Tobias, president of the NTEU, said "I don't see it as a raid" on AFGE. "We've been pretty successful taking the lead on issues important to federal employes for many years. We're just continuing to do what we've been doing.

"As a matter of fact," he said, "one of the ways available to represent federal employes is through litigation."

Federal unions are prohibited from bargaining over wages and cannot strike.

Through litigation, he said, "you can solve problems you couldn't otherwise solve. Sometimes it's the only arena available."

Diane Childers, communications supervisor for AFGE, said, "You can decide for yourself whether this is a cheap shot for them to say 'we got our material out before you.' "

The AFGE said it had won a suit in the D.C. Court of Appeals on June 26 against regulations promulgated in 1985 that essentially denied GS-11 employes overtime by "presuming" they were in exempt positions. In going into court yesterday, AFGE said it is "building on the federation's recent major legal victory."

"We don't feel we let them get the jump on us," Childers said. "We didn't want to go into just any court, but to have the best case in the most favorable area."