Bedeviled by a loss of jobs to nonunion plants that have sprouted like mushrooms across the Sunbelt, labor unions are exerting mounting pressure on big corporations to take a hands-off policy toward unionization of these new facilities.

In a related move, they also are trying to get job security protections that make it more expensive for companies to shut down old plants and more difficult to keep unions our of new ones.

These efforts started in the auto industry and have spread to rubber, electrical manufacturing and other industries. They are aimed both at protecting organized labor's base in the industrial North and at establishing a new beachhead for unions in the South.

As a whole, the campaign represents a departure from the traditional, bargaining-table fare of wages, hours and benefits - reflecting the fact that unions' negotiating leverage for these essentials hinges on their command of the work force. A "matter of survival," as one union president put it recently.

It also reflects the labor movement's difficulties in gettting Congress to do the job for them, as governments of many other industrialized coutries have done in greasing the union organizing machinery.

Last year, Congress cold-shouldered a comprehensive labor law revision bill that would have made it more difficult for employers to fight unions. This year, unions are pushing for statutory protections against plant closures but without much hope for radical remedies any time soon.

The political climate, reinforced by savvy corporate labbying, has prompted a number of unions to pursue these broad goals in the fine print of their individual contracts, where their clout can be focused more clearly.

The United Auto Workers led the way three years ago with a breakthrough agreement with General Motors Corp., under which the giant automaker agreed not to interfere with the union's attempts to organize its new plants, many of which were being located in the historically nonunion Sunbelt.

When trouble arose in execution of the so-called "neutrality pledge," the UAW warned GM that it was inviting an explosive confrontation if it continued to oppose the union's southern organizing efforts. The result was GM's agreement last September to a hiring policy for new plants that gives preference to card-carrying union members, along with an explicit reiteration of the company's neutrality toward UAW organizing drives.

Now the UAW is seeking in its Big Three contract bargaining to expand on the GM agreement in several important ways, including relocation allowances, company-paid retraining and automatic certification of the union as bargaining agent when new plants duplicate products made at previously organized plants. In other words, the company could wind up paying dearly to close down an old unionized plant - and then wind up nose to nose with the UAW again at a new plant.

Perhaps even more significantly, the UAW-GM agtreement began a chain reaction through the labor movement. Only last month, the United Rubber Workers, a smaller and weaker union, won a neutrality pledge for tire plants in its contract with Goodrich, creating the pattern for the whole rubber industry.

The pledge came easily for Goodrich, which has no nonunion tire operations, but could mean big gains for the union's membership rolls as a whole, which have been eroded over the years by plant closures and expansion of nonunion operations. Uniroyal and Firestone have agreed to the pattern; Goodyear has balked at the nentrality pledge, among other things, and still isnegotiating with the union.In all, the four companies have six unorganized plants, including their newest, biggest and most profitable.

The URW also won some new protections for workers when a plant closes, including a requirement for prenotification and negotiations over the shutdown, full pension after 25 instead of 30 years and 24 months of medical and other insurance coverage.

Electrical unions also sought a neutrality pledge and a broad array of plant closure protections in recent contract negotiations with the General Electric Co. but made no headway on these items, according to union sources.

Although the implications for whom growth in the South are still unclear, some labor relations experts contend that inroads by big industrial unions could build a sounder base than some better-publicized organizing efforts pegged to shakier industries such as textiles.

But the problems remain formidable even for powerful unions such as the UAW because resistance stems not only from corporate sources but also from deep-rooted cultural attitudes.

The UAW has moved slowly within GM in the South, picking up only one small plant in Louisiana since the September agreement. It now faces a major test in organizing a huge new assembly plant in Oklahoma City.

On the other hand, the importance of corporate neutrality can be seen in the resistance the United Steelworkers union has met from Tenneco Inc. in trying to get a contract with its New-port News (Va.) Shipbuilding and Drydock Co. subsidiary, even after workers voted to bring in the union.

"I think they're on the right track (with the neutrality agreements)," said a sympathetic industrial relations expert, "but I sure wouldn't break out the champagne quite yet.ak out the champagne quite yet.