Transportation Secretary Brock Adams joined a coalition of handicapped Americans yesterday in an attempt to rescue the embattled "Transbus" program from challenges by a powerful Congressional committee and the bus manufacturing industry.

Transbus, billed by the federal government as "the bus of the future," is a federally proposed design for big city buses with costly special features, such as wheelchair ramps and low boarding steps. It is intended to provide improved transit service in urban areas for elderly and disabled passengers. The plan has stirred intense controversy within the transit industry.

At the center of the latest battle is an emendment to a transportation authorization bill that was approved in May by the House Public Works and Transportation Committee. The amendment would require Secretary Adams to "reevaluate" his previous order requiring bus manufacturers to shift to building Transbuses next year.

Adams announced more than a year ago that all normal-sized buses purchased by local transit agencies receiving federal subsidies after Sept. 30, 1979, would have to meet Transbus specifications. The order would affect virtually all big-city bus systems because major transit agencies normally rely on federal aid to finance 80 per cent of the costs of buying new buses.

Adams voiced his opposition to the House committee's Transbus amendment as he spoke at a midafternoon rally yesterday sponsored by the American Coalition of Citizens with Disabilities. Speaking from the steps of the Cannon House Office Building, Adams asserted that his Transbus order should not be "extended or studied any more."

General Motors Corp., one of the nation's three major urban bus manufacturers, reasserted its opposition to Transbus yesterday. In a statement by GM President E. M. Estes, the company termed the government's Transbus requirements for low steps and ramps as a "serious mistake" because such ramps would be "too steep for many - perhaps most - handicapped people too use."

George Prytula, a spokesman for Grumman Flxible buses, said Flxible favors a reevaluation of the Transbus program because of continuing uncertainities over design features. "We've always been for Transbus, but we want them to make sure what they want," he said.

AM General, an American Motors subsidiary, recently announced that it would stop building standard urban buses partly because of what it described as "confusion" over the pending federal bus regulations.

The mounting uncertainties over the prospects for Transbus have left American transit agencies, including Washington Metro system, in doubt over how they should proceed in ordering new buses to replace obsolete vehicles in their fleets. Metro's board of directors is scheduled to debate this issue at a board meeting today.

Amid the Transbus controversy, moreover, bus manufacturers and transit agencies have already taken steps to redesign their buses to improve service for handicapped and elderly riders. Washington's Metro system, for example, has recently ordered 281 new Flxible buses, including 150 outfitted with wheelchair lifts for handicapped passengers.