A thrice-repeated but unsubstantiatedindictment of Argentina by Assistant Secretary of State Patricia Derian testifying publicly before Congress reveals the shattering impact on U.S. foreign policy of human-rights crusading.

Under questioning Aug. 9 by the House inter-American affairs subcommittee, human-rights chief Derian responded with language seldom used by one friendly power to another, accusing the Argentine regime of killings, kidnappings and torture. So harsh was her language that the State Department tried to expunge it from the record. But apart from being undiplomatic, there is considerable doubt of accuracy.

The impact could prove tragic for U.S.-Argentine relations and perhaps for Argentina itself. Moderate and pro-U.S. elements within the Argentine junta have been weakened; deterioration of the U.S. position in the strategic southern corner of South America has been accelerated.

This cannot be dismissed as merely unfortunate ardor by an idealist unfamiliar with diplomacy. Deterioration of U.S. relations with Brazil, Chile and now Argentina too closely follows the scenario of Latin American specialists in the Carter administration who privately predicted the human-rights crusade would foster the left in the Western Hemisphere.

Concern that the human-rights tail is wagging the foreign-policy dog is spreading in both the administration and Congress. On Capitol Hill, growing attention is paid to the imminent loss of up to $1.4 billion in sales to Argentina - including $620 million in Export-Import Bank transactions.

So, when Derian testified Aug. 9, she was asked by subcommittee chairman Gus Yatron (D-Pa.) why her human-rights office recommended against the Export-Import Bank loans. Even Derian's own aides were taken aback by her answer.

"The reason for our advice was the continuing violation of basic human rights by Argentina. The systematic use of torture, summary execution of political dissidents, the disappearance and the imprisonment of thousands of individuals without charge, including mother, churchmen, nuns, labor leaders, journalists, professors and members of human-rights organizations."

Moments later, Derian repeated her indictment, adding "kidnappings" and "unwarranted killings" this time. In her soft Mississippi accent, she read the litany a third time, concluding. "We see nothing to indicate that there is a genuine trend toward human rights."

Horrified officials in the State Department's Inter-American Bureau crossed out Derian's indictment in the transcript, but it was too late. Word had gone to Argentina, where Pat Derian has become a household word. The U.S. ambassador in Buenos Aires was called in for a stiff protest; the Argentine ambassador in Washington was called home.

The tragedy is that Derian's outburst may well weaken the junta's relatively moderate elements headed by the president, Gen. Jorge Rafael Videla, and strengthen the extreme right. While President Carter's human-rights admonitions probably have improved conditions in Argentina, the point of diminishing returns has been reached.

Apart from undiplomatic ravages, was Derian telling the truth? Her office insists the Argentine junta has "executed 3,000 persons since seizing power in 1976 and at least another 5,000 persons are missing. But those figures come from private sources of dubious reliability. U.S. government bureaus with vastly more experience than Derian's say the figures cannot be verified and seem inflated.

Actually, the junta, in confronting bloody far-left revolt in 1976, used an iron fist to prevent a communist takeover. But many killings and kidnappings are traceable to rightist paramilitary groups not under government control and should not be counted as government "executions." Moreover, objective observers agree that Argentina's human-rights record has improved markedly in the past year.

Those nuances are disregarded by Derian's office. While unable to draw distinctions between moderates and extremists inside the junta, the human-rights crusaders claim that military rule in Argentina cannot last much longer and that the United States should disengage from the junta. Such a prospect is rejected by Latin American experts, whatever their ideology.

But behind the naivete, a pattern emerges. Before Carter took office, a prediction was made by Brady Tyson, now with the U.S. mission to the United Nations, that human rights would be used to support revoluntionary forces in the hemisphere. An identical prediction was made privately last year by Robert Pastor of the National Security Council staff. Derian's undiplomatic incident could help fulfill those prophecies.