Labor unions' share of the total American work force has fallen below 20 percent for the first time since World War II, although union membership reached an all-time high of more than 20 million last year.

The apparent anomaly arises from the fact that union gains failed to keep pace with the growth of the work force as a whole, principally in the service sector as opposed to manufacturing jobs.

According to preliminary figures gathered by the Bureau of Labor Statistics in its biennial survey of labor organization membership, unions reversed their 1974-76 decline over the last two years and picked up 604,000 more members for a total United States membership of 20.2 million during 1978.

But, at the same time, the work force grew by 5.6 million, with the gains coming in service-type jobs that unions, with their traditional crafts and industrial base, have found hardest to organize.

As a result, unions' share of the total U.S. labor force continued its gradual 25-year decline from 25.5 percent in 1953 to 19.8 and 19.7 percent in 1977 and 1978, respectively. It was 20.3 percent in 1976.

The unions' difficulty in organizing both public and private-sector service employes, many of them white-collar workers, is compounded by the shrinkage in organized labor's traditional manufacturing base.

The BLS figures show that union manufacturing jobs declined from 8.5 million to 8.1 million over the last two years, for an overall decline of more than 1 million since 1974.

Although the slack was more than made up by gains in government and private-sector service jobs, most of these gains came at the expense of employe associations many of which have become unions in all but name. In other words, they were not newly organized workers.

For instance, the American Federation of State, County and Municipal Employees (AFSCME) continued to hold top place among the unions for recruiting new members, but more than 200,000 of its 270,000 new members came in one bite from New York State's Civil Service Employees Association.

As a result, the ranks of employe associations dropped sharply from a high of 3 million in 1976 to 2.6 million in 1978. Taken together, union and employe association membership gained only marginally in raw numbers, increasing by 136,000 to 22.8 million, but dropped as a percentage of the work force from 23.4 to 22.2 percent.

Nevertheless, the fastest growing unions were nearly all service-oriented, including retail clerks, teachers and communications workers as well as state and local government employes.

Of the eight unions that gained 100,000 or more workers over the past decade, the International Brotherhood of Teamsters, the biggest and once the fastest-growing, picked up few members in the last two years. The United Steelworkers, which is a prime example of a large industrial union plagued by a shrinking jobs base, was the only one of the eight that lost members over the last two years.

Among the biggest membership losers over the past two years were the carpenters, hotel and restaurant workers, paperworkers, railway and airline clerks, rubber workers and painters unions.

In a Labor Day radio address, AFL-CIO President George Meany tacitly acknowledged unions' difficulty in organizing outside of their traditional strongholds but predicted they will make major gains in organizing professionals, office workers and farm workers during the 1980s. While noting that the concerns of white-collar workers "may seem to differ from those of construction or industrial or public workers," Meany said, "in the '80s, I am confident there will be a strong growth of unionism among groups previously considered not interested in union representation.