Leaders of two unions that have a major stake in the defense industry yesterday joined a coalition that urged President Carter to fulfill his campaign promises and forgo big defense spending increases in favor of domestic needs.

The officials representing the United Auto Workers and the International Association of Machinists unions acknowledged that their members benefit from military spending boost.

But they and other coalition leaders, many early Carter campaign supporters, called a news conference to publicize a study by two former Pentagon officials, Townsend Hoopes and Herbert Scoville, who are longtime advocates of slimmer defense budgets.

Their study said that with selective reduction of forces and cancellation of certain new weapons systems, the Pentagon could lop about $10 billion a year for the next four years off spending increases.

Gene Cassaris, legislative representative of the UAW, noted his union's endorsement of the study and said, "There are amounts in the defense budget that could be put on the domestic side."

He and George Poulin, general vice president of the machinists union, stressed that they weren't urging cuts that would put their members in the defense industry out of jobs.

Poulin said, however, "We can cut back armaments without damaging our defense posture." The savings could go toward creating jobs in the domestic sector "so our members aren't out of work."

Ron Brown, vice president of the National Urban League, said, "We were pleased when Mr. Carter said during the campaign that he was going to slash the defense budget, we thought he was going to reviltalize cities, Put Americans back to work and provide health care and welfare reform. Now we're very concerned. We have not seen evidence that the commitment will be fulfilled."

Brown added, "No longer can we both guns and butter. We believe we should choose butter."

Syracuse Mayor Lee Alexander, president of the U.S. Conference of Mayors, cited a Washington Post report Sunday saying Carter was planing a $40 billion increase in spending outlays for fiscal 1979, which starts next Oct. 1, and that $9.3 billion of the increase would be for defense.

That, combined with Carter's desire for a balanced budget by 1981 and his proposal for a $25 billion tax cut, "indicates devasting consequences for urban America," Alexander said.

He said the coalition - which also includes leaders of the American Federation of State, County and Municipal Employees and the National Education Association - met yesterday morning with James T. McIntyre Jr., acting director of the President's Office of management and Budget, to voice their fears that domestic spending would be shortchanged.

Alexander said McIntyre "assurd us that the growth of the defense budget is not fixed in cement - that it's still under review."

However, an OMB source said later that "final decisions have beed made" on the defense budget but he would not give details.

The OMB official quoted McIntyre as telling the coalition leaders that Carter's budget "will go a long way toward resolving the domestic problems" that they raised McIntyre also noted that there are many overlaping domestic progams and that the administration want to end duplication and find ways to make the programs work better, the source said.

He said Edward Jayne, associate director for national security and international affairs at OMB, told the coalition that some programs that Hoopes and Scoville want cut are not in the fiscl 1979 budget.

By holding the news conference after their meeting with OMB officials, the urban, labor, and education leaders seemed to be reviving a coalition that started four years ago to fight President Nixon's impoundment of funds that Congress had appropiated for domestic programs.

That coalition set up a Washington organization called the Council on National Priorities and Resources, which has been dormant since Carter took office. The council sponsored the Hoopes Scoville study.

Hoopes said the sudy's recommendations are based on "our judgement . . . that the overall U.S. Soviet balance is stable" and that the Soviets are "acutely aware of the risks of nuclear war and immensely impressed by American technology."

The study called for canceling the MX mobile intercontinental ballistic missile, the MK 12-A warhead and NS-20 guidance system, the development of a maneuverable re-entry vehicle, and the Trident II missile.

It also called for personnel cuts in the Army and Marine Corps and opposed expansion of the 22-wing Air Force to 26 wings.