"LINKAGE" IS BACK, kind of, under the patronage of Zbigniew Brzezinski, seconded yesterday by his chief, Jimmy Carter. We refer to the doctrine that holds, in its most common variant, that if the Russians act badly in third countries, the United States will withhold its cooperation in some other policy area in which the Russians have special interest. Endorsed by the previous administration until it became impractical to apply, this doctrine was formally repudiated at the outset by this administration. But Mr. Brzezinski and Mr. Carter, watching with growing dismay the Soviet intervention in Ethopia and being no more able than anyone else to figure out a way to stop it, have now doubled a long way back.

The Soviet intrusion, Mr. Brzezinski says, "will inevitably complicate" both the negotiation and ratification of a new strategic-arms agreement. "The two are linked because of actions by the Soviets," adds Mr. Carter. "We don't initiate the linkage."

We wish the White House had let sleeping linkages lie. Analytically, it's right that Moscow's Ethiopia operation nourishes hard-line American sentiment and hurst SALT. But analysis, when it comes publicly from the president and his national security adviser, becomes policy, their no-hands pose notwithstanding. To remark on the connection is to endorse it, to lend some of the administration's authority to making it come true.

That, we submit, is unnecessary, even foolish. The Kremlin's Ethiopian power play presents problems to Washington, but those problems are properly defined and necessarily worked out in terms of policy in africa. The situation in Ethiopia has deteriorated so far, from the American viewpoint, that there may be no way to restore it. What can be done is to limit the damage: to see to it that Somalia's integrity is protected, to find the slow path back toward normal relations with Ethiopia, and to try to avoid "another Ethiopia" in Rhodesia. To hitch SALT to Ethiopia is to burden SALT without helping in Ethiopia. How, later, will the administration back off?

American hard-liners may be glad to hear the White House talking tough to the Russians, though if the talk fortifies them in their resolve to oppose an arms-control agreement, the administration may have occasion to think twice. People in the middle may also be glad, for another reason: They are already extending themselves to vote for the Panama Canal treaties, and they cannot welcome being called upon twice in an election year to support an extremely controversial international agreement. So, in effect if not intent, the White House warning may be an exercise in bone-throwing to the right and the center alike.

The fact remains that SALT is in plenty of trouble without Ethiopia's being dragged in. We do not see how the new declarations add to either the United States' leverage on the Russians or the administration's leverage on its Senate critics. Of course, the administration may be raising the Ethiopia issue to distract the Russians, and perhaps the American public, from its failure so far to negotiate an arms-control treaty. But frankly, we doubt it. We don't think the president and Mr. Brzezinski are being devious. We think they're mixed up as to their purposes. Instead of supplying grist to the anti-SALT mill, they should be protecting the abiding American interest in SALT from the fallout of a third-country flareup in which the Russian presence, however troubling in its implications, supports a principle -- territorial integrity --control agreement is far too important to the United States to be held hostage to the Ethiopian dispute.