The good news for organized labor is that the wages of the nation's union members are an estimated 33 percent higher than those of other workers, but there has been a 2.7 million-person decline in union membership.

Those are among the key findings of new surveys by the Bureau of Labor Statistics showing that union membership declined from 23 percent of the work force in 1980 to 18.8 percent in 1984, the lowest level in recent years. Union membership among nonagricultural workers reached a high of 35 percent in 1945.

The number of union members dropped from 20.1 million to 17.4 million since 1980 according to the BLS, a loss of 13 percent, while the civilian work force expanded almost 5 percent. Although membership was pegged at 17.4 million, union contracts covered 19.9 million workers.

The study documents the decline in union membership brought on by automation, by massive layoffs in heavily unionized industries such as automobile and steel, by the increasing success of employers' antiunion efforts, and by the sharp growth in service industries that unions have had little success in organizing, according to labor experts.

While losing hundreds of thousands of members through plant closings and layoffs, unions have been largely unsuccessful in organizing many of the new, smaller enterprises in health care, high technology and education. The loss of membership has also meant shrinking treasuries, rendering unions unable to devote more money to organizing drives, which have become increasingly expensive as employers have fought more legal battles to thwart unions.

"There's no surprise in these statistics. If there's any surprise at all, it's that we were afraid the figures had fallen even further," said AFL-CIO labor economist John Zalusky. The AFL-CIO, a federation of 96 labor groups, has about 13.5 million members but does not keep data on membership of other unions.

The most surprising finding to BLS was that union membership declined although the economy created 5 million new service-sector jobs since 1980, said a BLS economist who asked not to be identified.

The data, based on a monthly survey of 60,000 households, is the first BLS estimate of union membership made since 1980 because the bureau suspended its count of union members to save money in the first year of the Reagan administration.

Zalusky said the AFL-CIO thinks that the data is misleading on the percentage of work force belonging to unions, because many workers are self-employed, unemployed or are supervisors ineligible to join unions. The AFL-CIO's data is based on the "organizable" work force, which the federation estimates is 26 percent unionized.

The BLS household survey showed that the median weekly wage for nonunion workers was $303; for union workers it was 33 percent higher, $405. In 1983 the median union wage was 35 percent higher than that of other workers.

The wage advantage of union membership varied markedly, however, for women and minorities. Unionized women earned a median weekly wage of $326 compared with $251 for nonunion women. Black union members earned $357 while other blacks earned $236. Hispanic union members earned $351 compared with $238 for nonunion.