The differences that have emerged among top administration officials on how to deal with the kremlin may have been "vastly exaggerated," as some of them contend, but they are nevertheless real.

President Carter's national security adviser, Zbigniew Brzezinski, believes that unless the Kremlin checks the Soviet surge into Africa, the threat of "linkage" should be used to make it clear that both the present negotiation and the future ratification of a strategic-arms agreement could be endangered. Secretary of State Cyrus Vance believes that no such threat should be used. President Carter says the United States will not initiate the use of linkage, but explains that the Soviet conduct in Africa may cause Congress to refuse to ratify a SALT agreement - though he does not go so far as Brzezinski in saying that the current negotiations, too, may be affected.

All three agree that regardless of whether Washington brings in linkage or not, the militant Soviet posture in Africa may in the end harm SALT. They know that it could revive in Americans the distrust of Russia that would make it difficult to achieve the kind of arms agreement, with its inevitable compromises and concessions, that would be acceptable to both sides. But the premise on which must of this argument between contending factions rests is wrong.

It is wrong because the Kremlin's interest in SALT is no longer strong enough to give Washington the leverage it once had. In the past Brzezinski often maintained, correctly, that the Kremlin wanted SALT more than Washington did. Whether Moscow really needed it more was another question - but for its own political reasons it wanted it, and badly at that. In some ways Moscow's desire for SALT could be compared to its wish for trade and credits from the United States. It did want them badly, but when the political price proved too high, its desire cooled off.

The turning point on that issue coincided in Moscow with the illness and political weakness of Leonid Brezhnev, who could no longer muster the politburo votes for some of the concessions to the United States that he might have been willing to make. He then went on to recover his health and political strength - only to show, in recent months, a decline that must have once again affected his decisive role in the policy-making process.

Brezhnev's personal commitment to SALT, his association with it for a number of years, his vision of it as a monument to his period in office are beyond doubt. His ability to make that vision come true at a time when his political control is less than complete, when contending factions, in Moscow are thinking ahead to a future without Brezhnev, must be seriously in question.

That is only one of a number of reasons why the Kremlin may no longer want SALT as badly as it once did. It may also have persuaded itself - as have some SALT supporters in the United States - that for technical and strategic reasons, an arm-control agreement is no longer as vitally necessary as it was. Some experts on arms control have long argued that the security of both countries could be ensured by each doing what it thought was necessary for its own defense, while seeking not to provoke the other - because it would indeed be in the interest of each not to provoke the other.

Their argument leaves many questions unanswered, but it has serious attractions to the military-industrial complex in both countries.

But if Brzezinski may be wrong about the SALT-Africa linkage, he is certainly right about th seriousness of the Soviet threat in the Horn, and about the need to do something about it. Whether the Russians have a grand design for Africa, as was argued in last week's column, or are merely blundering deeper and deeper into the continent, drawn by Africa's irresistible opportunities for gain and for mischief, is immaterial. The threatening progression from Angola, where they used only the Cubans, to Ethiopia, where Soviet soldiers and massive arms supplies are arriving (at, admittedly, the invitation of the Ethiopian government) is self-evident.

The Kremlin is establishing a pattern, and a principle, that would make further progression possible, and enable it to intervene on the side of the Rhodesian guerrillas, without the invitation of a government. When the Rhodesian struggle spreads to South Africa proper, as it is almost certain to do, sooner or later the Kremlin would have even stronger reasons of its own for entering the fray.

The existence of a properly constituted Communist Party in Africa, the presence of an urban proletariat, the temptation to control the gold and uranium and the highly developed industrial assets of South Africa, its role as the political and strategic keystone of the continent - all these would provide ideological and material arguments for the activist faction in the Kremlin. It would want to take the progression and pattern established in the earlier Soviet interventions, in the absence of effective U.S. counteraction, to its natural conclusion in South Africa.

That is why the White House must find a way to stop the Russians before it is too late - and that goes not just for Brzezinski, whose natural inclinations lie in that direction anyway. It goes also for Vance, whose talents - admirably suited and necessary as they are to the development of a long-term foreign policy - should be brought to bear more forcefully on a crisis that may yet come to interfere with the unfolding of the policy he favors.

And it goes, most of all, for President Carter. He has allowed Brzezinski to act as his spearhead in the matter of linkage. But Carter may find that both the domestic and international fallout from Soviet actions in Africa may do serious damage to some of his own plans - including any plans he may have for next presidential election campaign.