WHEN THE United States was asked whether it would let in the shah of Iran, the administration said yes but not now: There would be security problems for the shah in this country and for Americans in Iran. When Mexico was asked, and when it was warned by the revolutionary government in Tehran that admitting the shah would affect official relations, a Mexican spokeman was authorized to say: "We are not going to allow any government or country to dictate our policy." The shah is now in Acapulco. Mexico did not in effect relieve the Iranian authorities of their duty to keep the mob from sacking the Mexican Embassy in Tehran. Nor did it dismiss its own responsibility to see that visitors receive appropriate protection. Mexico has a long tradition of openness to political refugees. By maintaining this tradition, it does the right thing and deters blackmail.

The adminstration's equivocation is unbecoming. The shah is, after all, the same person whom Jimmy Carter personally embraced on his own trip to Iran. The ugly side of his rule was a familiar story then and the only change in it that developed after the Carter visit seems to have been that the shah, partly in response to Mr. Carter's counsel, improved his performance on some aspects of human rights. In Washington , meanwhile, Mr. Carter was asking Congress to remove from American law the old and artifical ideological definition of political refugee and adopt a new definition stating simply that a refugee is someone outside his country who can't go home because of fear of persecution. The shah is not your ordinary refugee but, in spirit at least, he seems to fit the Carter definition well.

If self-respect and duty are conceptions a little bit too difficult for Mr. Carter to weave into his treatment of the shah, there is a practical consideration. For reasons that do not have to be gone into here, the United States regularly finds itself in the position of trying to persude heads of embattled governments to leave power. It has done this in recent years to the former leaders of Vietnam and Cambodia as well as to the shah; it is currently doing it to Nicaragua's Anastasio Somoza, it may yet do it to Zimbabwe Rhodesia's Ian Smith, or somebody. To urge a leader to step down and then to deny him refuge is an inconsistency that could blunt an important tactic of American diplomacy. It is also, of course, shameful.