The proposed breakup of American Telephone & Telegraph Co. could scuttle national bargaining agreements and labor stability within the domestic telecommunications industry, two union leaders said yesterday.

But labor peace could be preserved if Congress drafts and approves job-protection guidelines covering nearly 700,000 unionized workers in the Bell System, the union officials told the House telecommunications subcommittee.

"It took our union about 35 years to secure national bargaining, which has been a key element in the high degree of labor peace within AT&T," said John C. Carroll, executive vice president of the Communications Workers of America, which represents about 525,000 of the unionized Bell workers.

"Today, we see the probability of a . . . split in our bargaining, which will, without a doubt, have a serious effect on our . . . members for some time," Carroll said.

CWA has had national contracts with AT&T since 1974. Similar agreements have covered about 175,000 other Bell System workers represented by the International Brotherhood of Electrical Workers and the Telecommunications International Union.

National bargaining means the unions negotiate a "master contract" affecting their relationships with AT&T and all of its subsidiary units. But under the Jan. 8 divestiture agreement between the Justice Department and the telephone company--which must be approved by a federal judge--AT&T has promised to shed 22 local telephone companies with assets of about $80 billion.

As currently drafted, the divestiture would affect about 235,000 CWA members "who on divestiture day begin working for some new employer or employers," Carroll said. Depending on how and when the court acts, divestiture day could coincide with the expirations of current three-year labor contracts with AT&T. That would seriously complicate scheduled bargaining next year in the telecommunications industry, Carroll said.

Carroll and Anthony J. Salamone, administrative assistant to IBEW President Charles H. Pillard, said the divestiture could split bargaining three ways. One set of talks could be with the new AT&T, another with new subsidiaries created by AT&T, and still another group of negotiations with local telephone companies.

In the third category, the unions could find themselves negotiating numerous separate contracts--a development that could undermine the unions' ability to protect their members, the labor leaders said.

The current national contracts contain a "successorship memorandum of agreement" to protect the jobs and union rights of employes transferred to new affiliates or divisions of AT&T. But Carroll and Salamone said yesterday that those provisions do not apply to union members who find themselves in companies that have been fully separated from the Bell System.