In a notable departure from its intense propaganda blitz against China, the Kremlin has had little to say so far about U.S. congressional approval of most-favored-nation status for Moscow's archrival or the Pentagon's plan to sell defensive military hardware to Peking.

This silence points up what foreign observers here say may be the most serious casualty of the Soviet invasion of Afghanistan -- eradication of any Soviet hope, however fleeting, for some modest improvement in relations with its bitterest and most feared ideological foe.

In the aftermath of the invasion, which began one month ago, Moscow is faced with the indefinite postponement by Peking of normalization talks while watching the development of far more visible and possibly meaningful ties between China and the United States. Both countries have offered large new military and economic aid to Pakistan, which may strain Moscow's relations with India, even tghough its new prime minister, Indira Gandhi, has been markedly more pro-Soviet than pro-West and apparently will continue to be.

These swift reactions to the Soviets' Kabul coup are deeply upsetting to the Kremlin, which has more than 800,000 troops spread along the Sino-Soviet border, where there were sharp armed clashes in 1969 and other minor clashes since.

The stiff U.S. reaction, including a probable boycott of Moscow's cherished Summer Olympics, and the U.N. General Assembly condemnation of the invasion led by Third World nations -- where the Soviets fear Chinese influence -- have added to the leadership's discomfiture.

The Soviets' long-range goals have always been to seek better relations with the West so they could deal more forcefully with Peking. Now they find that the Afghan move has cut much of the ideological ground from beneath their attacks on the Chinese intervention against Vietnam last year as well as causing deep alarm and anxiety in the West.

The official Tass press agency has had virtually nothing to say about most-favored-nation status for China.

"This action by the Congress is another step in which Washington is endeavoring to play the China card against the Soviet Union," Tass said in its only reaction so far.