Unions that once fought their battles at the assembly lines, the mines and the mills of industrial America are now waging new kinds of wars for the hearts and minds of workers. The current skirmishes are occurring in the boardrooms and on the television screens and video terminals of a new generation of employes.

Organized labor is employing novel techniques -- including an increasing use of multimillion-dollar advertising campaigns and modern marketing strategies -- to try to reverse a generation of decline. In the months ahead, unions will face some major tests of those tactics.

Luring new workers to the labor movement and preserving the past gains of older, unionized workers are the twin agendas of labor. From the aging steel mills to the gleaming new office towers, and from state houses to the halls of Congress, unions are lobbying, organizing and advertising themselves aggressively.

In addition to Chrysler Corp., which is currently involved in tough bargaining with the United Auto Workers, other major industries facing a new round of negotiations in 1986 include the major steel makers, meatpackers, and American Telephone & Telegraph Co. And in each case, serious clashes are expected between employers seeking to cut costs in the face of ever-stiffer competition and unions seeking to reverse the trend of concessionary bargaining.

In Congress, the AFL-CIO and other unions are zeroing in on what they see as a golden opportunity to capitalize on the growing concern about the loss of an estimated 3 million jobs because of the nation's trade deficit. The labor federation believes it can play a major role in enacting some version of protectionist legislation, said AFL-CIO spokesman Murray Seeger. "People who used to label us as antediluvian are now on our side, including major corporations, executives and congressmen."

On the defensive side, labor also hopes to forge a winning partnership with corporations in fending off Reagan administration attempts to tax employe fringe benefits.

Organizing drives in health care, high-tech communications and other service industries including the government, will determine in large part whether labor unions will remain a major force through the 1980s and 1990s. In these efforts, unions are attempting to appeal to younger, nonunion workers by addressing concerns beyond wages and benefits.

Unions that once confined their efforts to bread-and-butter issues are now promising to be a stronger voice for workers concerned about career advancement, job security, work-place stress, VDT safety, pay equity for women, training and retraining and intangible yearnings for "job satisfaction."

The AFL-CIO is currently coordinating a nationwide organizing drive by five unions against Blue Cross-Blue Shield, aiming to recruit tens of thousands of clerical and professional employes in what has been an overwhelmingly nonunion industry. The effort is the labor federation's first coordinated campaign pitting multiple unions against a single employer.

New ground is also being broken in the similarly nonunion enclave of the nursing home. Two major unions, the Service Employees International Union and the United Food and Commercial Workers, have won elections at more than 60 nursing homes -- a 70 percent win rate -- in an ongoing 20-state campaign against Beverly Enterprises, the nation's largest chain of nursing homes. The Hotel and Restaurant Employees union is spending $2 million in an organizing drive aimed at hotels in four cities, including Washington, in which the union has hired dozens of college-educated organizers to recruit new members.

Typical of the growing role of public-sector unionism, at least six unions are currently pouring an estimated $10 million into a massive organizing competition in Ohio. There, a new collective bargaining law has opened the way for unionization of nearly 50,000 state employes and more than 100,000 county and municipal workers. Television advertising, direct-mail, and sophisticated polling have gone into the campaign by the American Federation of State, County and Municipal Employees, the Communications Workers of America, and other unions hungry to expand membership.

Polling conducted by various unions suggests that many workers, especially the younger ones, want unions to seek more cooperative rather than confrontational approaches to employers. The UAW, which is spending $2 million on advertising to enhance the union's image, is embarking on an innovative and closely watched experiment with General Motors in the Saturn project in Tennessee. The union has won job guarantees and a greater role in decision-making, while trading off concessions in the form of lower wages and more flexible job classifications, which GM sought.