In a challenge to President Carter and the Soviet Union, Sen. Frank Church (D-Idaho) said yesterday that the SALT II treaty should be ratified only if Soviet military forces in Cuba abandon their "combat role."

Since President Carter had failed significantly to alter the "status quo" in Cuba that he himself had declared unacceptable, Church said, "the Senate . . . must rectify and strengthen the American position."

Church, the chairman of the Foreign Relations Committee, suggested in a speech on the Senate floor that the Senate offer Moscow a choice "between the maintenance of a combat unit in Cuba and a SALT treaty."

Church proposed that the senate approve SALT II, which he called a good treaty, but make that approval conditional on an affirmatin by the president that "Soviet military forces in Cuba are not engaged in a combat role. . . ."

The president should also assure the Senate that those Soviet troops "will not become a threat to any country in the western hemisphere," Church proposed.Both these assurances should precede final approval of the treaty, Church said.

Church said the Soviets could comply with these conditions without any loss of face by disbanding their "combat brigade" in Cuba as quietly as they apparently assembled it in the first place. Church noted that the Soviets have never acknowledged the presence of their combat troops in Cuba, and would not have to do so to satisfy his new conditions.

The Carter administration is known to feel that it should be able to satisfy Church's new conditions on the grounds that the Soviet troops in Cuba are not engaging in a combat role, even if they are organized as a combat brigade. Church, however, made it clear in his speech yesterday that he would require a substantive change in the present situation. The potential for semantic confusion here seems great. Church said he has discussed his proposal with administration officials.

Church announced on Aug. 30 that U.S. intelligence had found a Soviet "combat brigade" in Cuba. At that time he said the Soviets should withdraw their combat troops from Cuba, and predicted Senate rejection of SALT II if they did not.

There was no demand for withdrawal in yesterday's speech, but Church reiterated his prediction that SALT would be beaten unless the Senate adopted his proposed conditions.

Church's proposal picked up three early cosponsors from the key bloc of southern Democrats whose support for the SALT treaty will be crucial. The three are Lawton Chiles of Florida, Walter (Dee) Huddleston of Kentucky, and James Sasser of Tennessee, all of whom have been counted as leaning toward SALT II.

In another development yesterday, Donald H. Rumsfeld, secretary of defense in the Ford administration, came out against SALT II in testimony before the Senate Armed Services Committee. In early 1976, Rumsfeld was instrumental in persuading President Ford to overrule his secretary of state, Henry A. Kissinger, and to choose not to pursue an early SALT agreement at that time.