President Reagan's hard line stance against the striking air traffic controllers could not have come at a better time for the nation's trade union leadership.

By refusing to negotiate and then firing the 13,000 striking members of the Professional Air Traffic Controllers Organization, Reagan has managed to escalate the confrontation from a basic labor dispute to a cause. And that may be just what the leadership of the AFL-CIO may need as it struggles to rally opposition to the president's domestic economic policies.

Until now, the trade union movement has been handed one humiliating defeat after another by Congress in the Reagan administration's so-far-successful effort to bring about radical change in government economic policy -- changes that threaten almost everything the labor leadership has stood for since the start of the New Deal.

The Reagan economic program of massive tax and budget cuts has swept through Congress with little more than token opposition despite lobbying efforts by labor and other traditional Democratic supporters to stop the program.

Unable to win support in Congress, AFL-CIO President Lane Kirkland has turned to the streets in an effort to regroup the traditional New Deal coalition. To that end, the federation is planning a giant Solidarity Day rally at the Capitol Sept. 19, a rally it hopes will do the same for the labor movement that the 1963 March on Washington did to galvanize the nation's civil rights movement.

"Solidarity Day will show those who have written off the labor movement that we have lost none of our driving force," Kirkland said earlier this summer in announcing the rally. The protest rally represents the most militant action taken by Kirkland since taking over as president of the federation in November 1979.

So far, however, there has been a great deal of skepticism, both in and out of the labor movement, about the potential success of the Solidarity Day rally. The biggest problem, critics say, is that the rally is being run from the top down rather than representing a grass-root protest movement.

Enter Reagan and the controlers. Until the president began carrying out his threat to fire the striking controllers, White House policy had been basically remote for much of labor's rank and file. But now, the president has taken actions that directly affect rank-and-file union members, and the appeal of the Solidarity Day rally could quickly take on new meaning.

With pictures of union members being led to jail in handcuffs and leg irons appearing in newspapers and television stations across the nation, Reagan has run the risk of rallying the trade union movement against the government.

But Reagan also runs another risk. He could lose the strike. Despite the firings, there is a good chance he eventually may have to settle with the controllers union. Whether it's police, fire or controllers, says a knowledgeable labor official, governments almost always are forced to take the strikers back with amnesty. If this should happen to Reagan, with all the government threats and warnings, it would be clear reaffirmation of labor's belief that if you have the power, you can strike.

And if the president holds firm, it could be several years before the nation's air traffic gets back to normal.