Speaking with that clarity of thought that comes with having decided not to run for reelection, Sen. Adlai Stevenson (D-Ill.) had the best of a (losing) arguement the other day for continuing the American embargo against grain sales to the Soviet Union.

By a furtive, unrecorded voice vote, the Senate went ahead anyway to cancel the embargo that was imposed, you will remember, as one way of protecting the Soviet invasion of Afghamistan. The issue must now be resolved by a House-Senate conference -- the House having voted heavily the other way. Farm belt pressure may yet prevail.

But stevenson had the right point. He freely conceded that the embargo may have been not only ineffective but at least as disruptive to this country as to the Soviet. (Much the same could be said for the Olympics boycott or the rush to revive registration for the draft.) But the Carter administion was trying to find some way short of armed intervention to signify its displeasure and demonstrate resolve by conveying national readiness to sacrifice something.

Thus to scrub the embargo, Stevenson aruged, would be to signal U.S. resignation to the Russian invasion of Afghanistan." And to do that -- and here's the curx of the Stevenson argument -- would be to "weaken a presidency already weakened by repeated congressional interventions in the conduct of foreign policy."

Stevenson is not talking about the Senate's traditional rights to advise and consent on treaties or ambassadorial appointments. Nor would he renounce the unquestioned right of Congress to use its powers of the purse to shape or restrain this or that foreign policy initiative by the executive branch.

The rightful power is there. What Stevenson was questioning is the wisdom of using it, at every turn along the way, to second-guess, overrule or otherwise confound the president's freedom to discharge his constitutional responsibilities for the national security.

He was not, I gather, talking just about the Carter presidency, but any presidency, and in particular any presidency since the Vietnam experience: the secrecy; the sudden, shocking surprises; the deception; the unacknowledged bombings and clandestine use of the Central Intelligence Agency with only the most token congressional oversight.

Given all of that, some swing of the pendulum -- some vigorous reassertion of congressional prerogatives -- was understandable. Such reassertion was an element even as the Vietnam War was winding down, in the clamp put on further use of American forces, and again when Congress barred resort to CIA intervention in Angola. It figured in the unsuccessful congressional battles to kill the Panama Canal treaty, ban the sale of military jets to Saudi Arabis, refuse the lifting of the Turkish arms embargos. All these were projects that President Carter thought critical to his conduct of foreign policy.

But nothing better illustrates this continuing conflict of interests (congressional and presidential) than the recent Senate failure by an eyelash to block the sale of nuclear fuel to India. Leaving aside the merits (which strike me as endlessly arguable as to whether the sale will advance or damage the unassailable purpose of non-proliferation), the transaction derives from a 1963 agreement between India and the United States. It was, in other words, a contractual obligation, good under its terms for 30 years. And never mind that it was consummated four American administrations ago.

What clouded the sale was passage in 1978 of the Nuclear Non-Proliferation Act, setting new and tighter rules for controlling shipments of nuclear fuel to countries that refuse to accept international safeguards against its conversion to military purposes. An admirable and sensible precaution, and undeniably the law of the land.

But not, as the Indians were at pains to point out, the law of India; a deal, they insisted, is a deal. And whatever you may think of India's record in this matter (maddening, just to begin with, that is what was often lost sight of: the issue of the sanctity -- or simply the worth -- of an international agreement undertaken in good faith.

You might call it the vacillation-in-foreign-policy issue, or the loss-of-confidence-in-America's-word issue -- to pluck from the Reagan/Republican bill of particulars against the Carter presidency.

Some days, the president's congressional critics get so caught up in the making of foreign policy that they clean forget just how much they contribute -- whether with a grain embargo or a nuclear arms sale -- to Vacillation and to the loss of confidence in American foreign policy.