As tensions grew yesterday in the air traffic controllers' mortal struggle with the Reagan administration, it was increasingly clear that the strike has become a psychological touchstone for frustrated employes in other federal unions, and could set the tone for government labor relations for some time.

"Whether or not it's legal to strike, if people get frustrated enough they will," said Greg Kenefick, a spokesman for the largest federal union, the AFL-CIO-backed American Federation of Government Employes (AFGE). The air traffic controllers, he added, "are in a way a stalking horse for federal employes' unions. . . . They're cutting a lot of new ground for the people who will come after them."

Union leaders yesterday were planning picket lines and weighing other possible actions to demonstrate their support for the controllers.

"It's a terrible situation. . . . It's almost as if we're spectators watching the Christians and the lions," Kenefick said. "And the lions are winning."

No matter who wins, it is unlikely that other major executive branch employe unions will be girding for a similar showdown soon, according to those familiar with the peculiarities of federal labor-management relations.

While there is increasing militancy among the federal rank and file, they say, most of the executive branch labor unions are far less organized than the tight-knit Professional Air Traffic Controllers Organization.

More than two-thirds of the executive branch's 2.3 million employes are represented by labor unions, not counting 660,000 Postal Service workers, compared with 22.2 percent of the private sector work force.

But not all federal employes who are represented ae dues-paying union members.For instance, AFGE, the largest federal union, represents nearly 700,000 employes, but only 250,000 pay dues.

The federal unions represent a vast array of occupations, from nurses and clerks to metal workers and machinists. Most work for the military, the Veterans' Administration and Treasury and Health and Human Services.

But there are significant differences between federal unions and their counterparts in the private sector.

Federal employes are prohibited by oath and by statute from striking and, with the exception of postal workers and a few others, are not allowed to bargain for pay.

Further, the government is in effect a "right-to-work state" in which workers can't be compelled to join and pay dues. In addition, their managers have no control over pay and no profits to consider.

The federal unions, however, have a significant influence on working conditions and grievances, everything from how the work is parceled out to who is promoted or fired.

And, despite the restrictions, they also have had considerable impact on federal pay and benefits, by lobbying in Congress instead of bargaining at the table.

In fact, said Donald J. Devine, the new head of the Office of Personnel Management, the unions "may have historically been more valuable as lobbyists than as pay negotiators," when it comes to determining pay.

Demoralized over the years by charges that they are overpaid, inefficient and can't be fired, and now beset by budget cutbacks and layoffs, federal workers are swelling the membership rolls gradually, according to union leaders.

The say they are concerned about confrontational tactics and "recalcitrant bargaining" by the administration, and that they are looking toward the day when they will have the kind of solidarity they need for effective action, legal or, if necessary, illegal.

The future of labor-management relations in the federal sector obviously depends a great deal on how the administration handles the current strike, Devine of the OPM said.

The law against strikes,he added, is meaningless unless the administration can make it clear that the costs will be prohibitive for any who violate it.

"But i don't expect the others will follow suit," he said. "I do know that most though the controllers were reckless in going out on strike."

Just as a successfuls trike by the controllers would send a certain message to the other unions, "the other side of the coin is that if the air controllers lose big, it's going to have an impact on federal labor relations" in the opposite direction, said Tony Ingrassia, a veteran Labor-management specialist at the OPM.