Secretary of State Edmund S. Muskie warned today that a U.S. policy turn against steady progress in arms control, with ratification of the SALT II treaty as its centerpiece alliance relationship throughout the world.

Seeking to draw a clear line of difference with the Republican Party and its presidential nominee Ronald Reagan, on an issue of major international importance, Muskie said that "the strength and unity of our alliances genuinely will be shaken" if the United States should swerve from arms control commitments which were undertaken with allied support.

Both Reagan and his party's platform oppose the SALT II agreement with the Soviet Union. Muskie has also charged that a GOP platform endorsement of the aim of U.S. military superiority is incompatible with the basic principle of arms limitation agreements with the Soviets.

Two days of speechmaking here and in Los Angeles were described by Muskie as the start of a nationwide campaign to take foreign policy issues to the public this fall, Muskie told reporters that he might travel as much as one or two days a week in this pursuit during the next several months.

This assumes that President Carter wins the Democratic nomination at New York next week and that Muskie will be speaking this fall as secretary of state. The former Maine senator continued to play down recurrent suggestions that he might lead the Democratic ticket if Carter's bid for renomination should fall.

Speaking today to a joint meeting of the Commonwealth Club and World Affairs Council, Muskie did not set out an adminstration timetable for moving ahead to Senate debate on the approval of SALT II.

Muskie said the Soviet invasion of Afghanistan, which led Carter and the Senate leadership to shelve plans for the debate early this year, "has made the task of ratification harder." He added, however, that ratification is no less important to U.S. national interests and to common East-West strategy involving the allies.

At the present time "the votes aren't there" to ratify the treaty, Muskie told reporters He expressed doubt that a post-election session of the lame-duck Congress would be a good time to move ahead. Thus, early 1981 would seem to be the earliest practical moment for ratification.

Muskie made clear that he is hoping to build the constituency for the SALT II treaty as well as to smite the Republicans in his public appearances this fall. He appears to have at least the tacit approval of Carter, who was given the West Coast speech text in advance. According to Muskie, Carter had praise for the speeches at this morning's White House breakfast of senior foreign policy makers.

As in the past, Muskie discussed the administration's recent arms buildup as well as his commitment to arms control. And for the second straight day, Muskie, in a slip of the tongue, spoke of himself as "secretary of defense."

A possible explanation for this recurrent slip involves suggestions that Muskie recently has come into philosophical or policy conflict with the Pentagon.

Speaking to reporters on his way west, Muskie hinted at disapproval of news reports of a recent revision in U.S. nuclear weapon targeting against the Soviet Union. Muskie had no part in the discussion leading to this policy change, aides said, but the State Department was consulted before he became secretary three months ago.

Asked today by a questioner in the San Francisco audience about possible conflict with presidential assistant Zbigniew Brzezinski Muskie responded that there have been "no confrontations" with the White House aide so far. He added that he had been impressed with the need for better coordination with various organs of government dealing in foreign polcy; and prominently mentioned the Pentagon in this respect.

After a lunch here with family members of U.S. hostages in Iran, Muskie told a news conference there had been "a lot of tears and some frustrations" voiced. "I told them to make me their target," he said.