A REQUEST BY 18 senators, backed by some 60 private organizations, to send emergency food aid to communist Vietnam and Laos puts President Carter's humanitarianism to a difficult political test. The international agencies report that the two countries are badly shot on wheat and rice, leaving it to others to argue whether the shortfall is due more to weather or political disruptions. But the two states are also on poor terms with the United States. The American Embassy remains open in Laos, though relations are minimal. There is no American mission in Vietnam, and talks to normalize relations have been nursed along slowly; on the American side there is no great inclination to press the pace while the Panama treaties are being debated. The espionage charges that led the State Department to oust Vietnam's United Nations envoy the other day indicate how brittle things are.

It will surprise no one that it was Hubert Humphrey who, just before he died, took the lead in organizing a Senate petition to Mr. Carter on thsi issue. He was aware that Congress has shown scant sympathy for the administration's effort at reconciliation with the communist victors of the Indochina war. Nor did he pretend that the food question, or even the larger reconciliation question, are priority concerns for the United States. But he did recall some relevant precedents in offering humanitarian aid to former foes, and he pointed out the few channels that Congress has left open by which a modest amount of assistance might now flow. Surplus food happens to be running out of the United States' ears right now. "Thank you very much for your thoughtful consideration in this matter," Sen. Humphrey and his colleagues concluded. President Carter has yet to respond.