Morton Bahr, president of the Communications Workers of America, yesterday predicted a nationwide strike against American Telephone & Telegraph Co. unless there is substantial progress by midnight today toward resolving disputes over wages and proposed changes in job classifications.

"Right now I am pessimistic. I think we have less than a 50-50 chance . . . . We are considerably further apart, much further apart than we normally are on the day before contract expiration," the union president said after a private meeting with Raymond Williams, AT&T vice president for labor relations.

Williams was not available for comment, but AT&T spokesman Herb Linnen said company officials "remain optimistic" that they will settle and avoid a strike by 155,000 members of CWA and 45,000 members of the International Brotherhood of Electrical Workers. Three-year union contracts expire at 12:01 a.m. Sunday.

"We have problems, but I don't get the sense that the problems can't be resolved with some hard bargaining by [Saturday] night," Linnen said.

AT&T has prepared supervisors to replace 38,000 long-distance operators temporarily in case of a strike. The company has said most customers should not be inconvenienced, because 90 percent of its daily volume of 33 million calls is connected automatically without operator assistance. Local telephone service would not be affected.

The most serious impact of a strike, according to industry analysts, would be in the highly competitive businesses of office automation and telecommunications switching gear. Bahr said AT&T could lose substantial business if regional telephone companies and other major customers are forced to take business elsewhere in the new competitive environment of the deregulated telecommunications system.

AT&T, which made a $1.6 billion profit in 1985 but did not meet its own projected profit goals, has proposed pay raises of 1 percent this year and 2 percent in each of the next two years, boosts that CWA has labeled "totally inadequate." The company also wants to eliminate its cost-of-living allowance and scrap a "piecework" pay system in which some 30,000 manufacturing workers get bonus pay of 15 to 25 percent if they achieve certain production targets, Bahr said.

Bahr, who was elected president last year and who has participated in AT&T bargaining since 1974, criticized the wage offer and faulted the company for waiting until the 11th hour in an eight-week negotiating process to propose major changes in job duties, among other things.

"What burns us," Bahr said, is that AT&T waited until last Saturday to present the union a 38-page proposal in which many of its 20,000 "systems technicians" would have job duties changed and, in some cases, pay reduced. "That's not fair. That's not the right way to do business," he said.

"This is not the way AT&T has operated . . . . In our entire relationship with AT&T, never would they come to the bargaining table without a year of discussion," he said. Bahr made his remarks during a luncheon with reporters and editors of The Washington Post.

Linnen said the union has been aware of the company's interest in changing job categories. Systems technicians are paid $646 a week for often highly skilled work, he said, but some less-skilled "techs" only "pull wires" and are paid far more than their counterparts at AT&T's competitors.

"A business will come to AT&T for an installation, and we quote a price, and that business says, 'Wait a minute, we can get it for a lot less elsewhere.' So we are trying to deal with that problem" by changing pay grades, he said.