The U.S. Circuit Court of Appeals here ruled yesterday that Congress violated the Constitution by granting tax exempt status to veterans' organizations that lobby while denying the same advantage to other nonprofit charitable and educactional organzations.

"The broad tax support provided to veterans' organizations for lobbying on any side of any issue they choose, and the companion restriction on lobbying by charitable groups and other . . . organizations, creates an unconstitutional disparity," Judge Abner Mikva wrote in the majority opinion.

The court did not rule that veterans' organizations must be denied exempt status. Instead it sent the case back to the District Court level with instructions to remedy the constitutional problem either "by restricting the tax benefits accorded veterans' organizations or by extending those benefits" to charitable and educational organizations engaged in lobbying.

If the outcome is to expand the tax break, the consequences could be significant. Many organizations that do not lobby out of fear of losing tax benefits might begin to get into the legislative process, and those that do engage in lobbying would be able to receive tax deductible contributions.

The case was brought by an organization called Taxation With Representation, which had been lobbying the executive branch and Congress on tax issues.

Mikva, in the decision which overturned rulings by both a District Court judge and a panel of three appellate court judges, said "even in the arcane intricacies of the tax code, the government cannot give special voice nor lend special ear to any person or group no matter how worthy their ideas or their credentials."

This view was challenged in a dissent by Judge George E. MacKinnon, who wrote:

"Veterans' organizations are essentially different in character from other charitable and educational groups. Congress has recognized that veterans' groups are unique entities having 'differing structures and differing purposes.' "