The story of the year-long U.S. grain embargo against the Soviet Union is the story of the Carter administration's life -- of its almost uncanny knack, in foreign affairs, for doing the right thing the wrong way.

The embargo, more so than the other two principal American reprisals to the Soviet invasion of Afghanistan (the Olympics boycott and the cutoff of technology), continues to make its point. Afghanistan, the United States was saying, wasn't its idea of business as usual. And so, in as many as it could contrive short of military force, the United States was going to suspend business as usual.

The problem was with the hype. Even some of the most loyal top administration officials will now admit that. "We created the wrong impression that somehow these measures could bring about the withdrawal of Soviet troops," says one. "They just weren't punitive enough for that."

The American reprisals did not bring an end to what Jimmy Carter, in one of his finer rhetorical flourishes, called "the most serious threat to world peace since the second world war." Thus, it was all the easier for Ronald Reagan to promise, in the heat of the campaign, to end the grain embargo because it "has hurt [American] farmers and has accomplished little or nothing."

That's a promise the president-elect ought to feel free to re-think -- and there are encouraging signs that he is doing just that. From the Reagan transition team there's talk of "reconsideration." The agriculture secretary-designate, John R. Block, who first talked of lifting the embargo "as soon as practical," is now saying it should be done at "the right time."

Not a conclusive shift, perhaps. But enough to provide running room for Reagan, as president, to demonstrate prudent flexibility, the large, strategic "global sense" his advisers promise -- and a willingness to accept realities.

The first reality is that the grain embargo exists. With the Olympic boycott a thing of the past, it remains the single most visible and dramatic expression of American protest -- of toughness, if you will -- in response to what was widely perceived to be a serious Soviet act threatening the peace.

A second rality is that while the embargo undeniably bears down on grain producers to the exclusion of other farmers and most of the rest of the economy, the damage to the American grain trade has been minimal (and is likely to continue to be) while the impact on Soviet food shortages has been severe.

American grain growers complain with some justice that the Soviets have been able to acquire much of the grain they otherwise would have purchased in the United States. But bad harvests have created a worldwide shortage; U.S. grain exports are at record levels. And the Soviets, meanwhile, have still fallen far short of their needs and have been forced to draw down heavily on reserves. Meat consumption in the Soviet Union has dropped off alarmingly.

Soviet President Leonid Brezhnev has publicly admitted to serious food shortages and given top priority to "improvement of the food supply." The incoming Republicans chairman of the Senate Foreign Relations Committee, Charles Percy, brought back from Moscow what he regards as convincing evidences that the embargo is having a punishing effect.

"They talk of it as loosening belts," he told me, "but what they mean is that they can't feed their own people."

Which brings us to the third reality: the Russians, a year later, are still struggling to establish their control over Afghanistan. No expert I have talked to believes they will withdraw their forces until they are satisfied that they will leave behind a Marxist, Soivet-oriented government able to maintain security.

While Vietnam is a weak analogy (Afghanistan is contiguous; the insurgents are lightly armed with little outside support), the battle reports have a familiar ring. They speak of "search and destroy" operations with helicopter gunships against elusive, resourceful, determined guerrillas; of whole areas beyond effective Soviet military control, or controllable only during the day.

The grain embargo, in other words, is not going to be decisive in Afghanistan. But then it never could have been. The question is whether, given all the realities, it is something that the United States should unilaterally yield up.

I find it hard to imagine the Reagan administration's cancellation of a valid American protest and throwing away what appears to be a valuable bargaining counter without -- in the spirit of "linkage" -- receiving something in return.