Union membership in the federal government has been dropping during the past few years while growing at the rate of 5,000 to 10,000 a month in the much larger market covering employees of states, counties and local governments.

According to the latest national union membership figures - as of late-1976 - from the Labor Department, 1.3 million U.S. employees belonged to unions. That was a drop of 91,000 from the federal union membership peak of 1974.

During that same time frame, 1974-76 membership at the state-local-municipal government employee level increased from 1.5 million to 1.7 million. Uncle Sam has about 2.8 million federal civil servants, postal employees and blue collar workers. States and local governments have an estimated 12 million to 14 million employees whose ranks are growing and become more unionized every day.

The nation's two largest unions are independents - the Teamsters and the National Education Association each have about 1.9 million members.

In the decade between 1964 and 1974, the Teamsters union was the most successful recruiter. But since has been replaced by the aggressive American Federation of State, County and Municipal Workers. That AFL-CIO union, also expanding into the federal side of government, now has more than 750,000 members and has been gaining about 5,000 a month for several years now.

Union membership growth in the federal government has slowed. Officials believe, because unions reached a saturation point within the white-collar sector and lost many blue-collar worker members because of layoffs and base closures by the Defense Department.

Although still the largest federal employee union with 285,000 members, the American Federation of Government Employees has lost about 40,000 members in recent years, mainly because of job cutbacks at Defense where the bulk of its members work. In recent months, AFGE membership has been increasing, but the earlier decline and the "saturation" factor are reasons why AFGE is considering moving into the armed forces. Results of a nationwide AFGE membership vote on the question of organizing the military will be made public Wednesday.

Federal and postal unions are pushing legislation in the House that would require employes represented by them who benefit from contract improvements to belong to the union or at least pay dues. Although postal groups bargain with the U.S. Postal Service for wages and fringe benefits, other civil servants have their pay set automatically and unions are confined to bargaining about working conditions, grievance machinery and other job-related aspects of federal employment.

Two federally-oriented unions that have bucked the stabilization or downturn in membership are the independent National Tresury Employees Union and the National Alliance of Postal and Federal Employees, a predominately black union, most of whose members in the postal service.

The Treasury group has expanded membership that once was limited to the IRS through a vigorous program of legally challenging pay raise amounts and working conditions. The Alliance has added members, with its excellent health benefits program for members and associates. But the Alliance remains relatively small compared to AFL-CIO unions that dominate the postal scene and already have as members about 8 of 10 rank-and-file workers.

Federal and postal employee unions are seeking major revisions in the Hatch Act, which has kept civil servants out of active partisan politics since 1939. The House has approved a revised Hatch Act bill that gives the unions most of the political freedoms they seek. The Senate still has not acted.

Revision of the act, union leaders believe, would help the federal work force increase its leverage with Congress and the White House, making it easier to win a union or agency shop system in government and making voluntary union membership seem more worthwhile to thousands of employees who have stayed out or dropped out of unions in recent years.