Twenty years ago, American workers voted to have unions represent them twice as often as they voted to keep unions out. Now unions are losing slightly more representational elections than they win.

Twenty years ago, 33 per cent of American workers in nonfarm jobs belonged to unions. Now fewer than 26 per cent do.

In 1956, the American Federation of Labor and the Congress of Industrial Organizations ended two decades of rivalry and united into one powerful labor force promising a concerted effort to put the union label on the America marketplace. Today that promise remains unfulfilled.

AFL-CIO President George Meany has never been as interested in organizing the unorganized as he has been in achieving results for the already organized, leaving the recruitment chores largely to individual unions.

For this and other reasons, unions are on a treadmill, puffing - some harder than others - just to keep from sliding further backward as the job market expands and changes.

But now the unions, Meany included, are talking of a major new effort to expand their ranks - principally but not exclusively in the sunbelt states along the southern tier of the country, where low pay and an anti-union climate have lured industries and thus jobs away from the unions' historic strongholds in the industrialized Midwest and Northeast.

The AFL-CIO recently set up a special organizing coordinating committee, and a number of individual efforts.

The stepped-up interest was triggered by a coalescing of forces rather than one single development, according to Al Zack, AFL-CIO public relations director.

Among them, said Zack, is optimism stemming from the arrival of a Democratic administration that the unions helped elect, a decline in union infighting, an infusion of new blood in top leadership ranks of many unions and a gradual mellowing of Southern hostility toward unions.

Already some gains have been made in the South, including a potentially far-reaching agreement by General Motors with the United Auto Workers last November not to fight unionization, meaning a hands-off policy toward organizing Southern plants.

Similar neutrality pledges have since been won from American Motors, Deere & Co. and smaller parts companies, and more organizing drives are expected soon, according to UAW spokesman Donald Stillman.

But the cutting edge of the Southern drive is the nationwide union-organized consumer boycott of J. P. Stevens & Co., the naton's second largest textile maker (after Burlington) and one of the premier union fighters.

The boycott campaign has enlisted the support of civil rights groups and is moving into full swing. Although it has yet to produce perceptible results, union leaders say they are confident of an eventual victory that will, as they put it, "unlock the door" to the South as a whole.

"It's not just symbolic," said Jacob Sheinkman, secretary-treasurer of the Amalgamated Clothing and Textile Workers Union (ACTWU). "Stevens IS the power structure "of the South's dominant industry, he said.

On top of this, organized labor, backed by an $800,000 AFL-CIO war chest, is mounting a major campaign to win congressional approval of basic changes in labor laws to make it easier to organize workers and win collective bargaining agreements.

These changes include repeal of legislation authorizing state right-to-work laws banning union shops, which all Southern states have adopted to help keep the unions at bay, as well as heavier penalties for labor law violations. They also seek expedited election procedures.

None of this will be easy.

J.P. Stevens has been outfoxing the textile unions since it was made their No. 1 target 14 years ago. The drive in Congress got off to a sputtering start when the House unexpectedly torpedoed labor's first bill for the year, a measure to expand unions' picketing rights at construction sites.

And the future was not brightened when Democratic leaders in both houses served notice that they won't even docket the right-to-work repealer. Moreover, the Carter administraion has refused to play by Meany's rules on appointments, some economic decisions, the minimum wage and import restrictions.

But the real problems appear to grow out of the labor movement and the economy in which it operates, as the experience of the last 20 years indicates.

Over the years, unions did add new members to their ranks, but they grew nowhere nearly as fast as the non-unionized sector of the economy. "It took a helluva lot of organizing just to stand still," said Zack.

Most of the big industries - autos, steel, rails - were organized early. As technology and imports reduced industrial jobs (Zack says imports alone cost 1 million American jobs over the past 10 years), work was expanding in non-industrial service areas outside the unions' traditional sphere of influence.

Labor leaders also cite legal impediments such as right-to-work laws and procedural delays in disputes that come before the National Labor Relations Board. They say the laws and the procedures spawned by them encourageresistance by anti-union managements.

"Instead of using goons, they're using high-priced lawyers," Meany complained recently. "A mugging is a mugging whether it's in a back alley or a courtroom," he said.

Stevens is Exhibit A in the unions' list of complaints.

The company has been cited 15 times in the last 14 years for labor law violations by the NLRB, and has been ordered to pay more than $1.3 million in back pay to aggrieved workers.

But even though workers elected ACTWU as their bargaining agent at seven of Stevens' 85 plants four years ago, the company still successfully resists signing a contract. It's cheaper to keep paying fines than to pay union wages, unionists charge.

Some union leaders also concede that many industries have become more skillful and sophisticated in their personnel policies, offering most of what unions offer, without union dues.

Labor's critics say unions, personified by their leadership, have become complacent, out of touch with the nation's changing work force and absorbed with their own power.

A number of labor-leaders, while rejecting most of the charges, worry about the image of unions and how it affects organizing.

"I see the polls and I'm appalled that we come in somewhere between Richard Nixon and used car salesmen," said William W. Winpisiner, incoming president of the International Association of Machinists. "The principal problem of organizing is the image of the labor movement, and the image comes from the top."

Zack concedes that labor's image is problem in organizing, blaming it largely on scandals emanating from the Teamsters. But the Teamsters have been successful organizers, recruiting even police officers.

Both sides agree on one major problem: the growth sector of the job market has recently been in the white-collar work force, and these workers have historically been difficult for unions to organize.

But it is among public employees - white-collar teaches as well as blue-collar garbage collectors - that unions have been making their only real gains in recent years.

If it weren't for public employee unions, whose ranks have more than tripledin the last two decades, organized labor would have been losing ground even faster it has.

Between 1968 and 1974, for instance, union workers in manufacturing declined from 9.2 million to 9.1 million, while unionized public employees grew from 2.2 million to 2.9 million according to the Bureau of Labor Statistics.

Moreover, once strictly professional associations like the National Education Association have become more and more like unions and grown in the process. The NEA's membership is now 1.7 million, up from just over 1 million 10 years ago.

Most other industrialized nations have maintained a higher level of unionization despite similar trends in the labor force.

According to the Bureau of Labor Statistics, the unionized percentage of the work force is 50 in Great Britain, 41 in West Germany, 35 in Japan and more than 60 per cent in Scandinavian countries.

Unions command 36 per cent of the nonfarm work force in Canada, according to Canadian figures. In Britain, Germany and Canada, the proportion of union workers is growing. These figures include professional associations.

Counting associations, the comparable figure for the United states was 29 per cent, as of the most recent BLS count in 1974.