IZVESTIA TOOK the campaign against Andrei Sakharov an ominous step further on Wednesday by almost -- but not quite -- accusing the Nobel physicist of treason. Mr. Sakharov, whose defense-related scientific work was done in the 1940s and 1950s, and who has not even held a security clearance since 1968, was accused of having "embarked on the path of direct betrayal of the interests of our motherland . . . [and] repeatly blabbed about things that any state protects as important secrets."

The strong but deliberately vague language reflects the Soviet government's ambivalence on how to deal with the unquestioned leader of Russia's intellectual and dissident circles. Apparently there are some who would have preferred a formal charge followed by a trial. The solution finally chosen -- internal exile to the closed city of Gorki only 250 miles from Moscow -- is a halfway step that is unlikely to work for long. While it may remove Mr. Sakharov from daily contact with fellow dissidents and foreign journalists, internal exile will not silence him altogether. In some ways, his voice may even be amplified; whatever snippets of his writing do get smuggled out will be seized upon with greatly increased interest. Mr. Sakharov has a long record of resisting intimidation and could well provoke the government into taking the next step.

Because of the now sharply constrained U.S. -Soviet relations, there are few types of leverage left that could be used to help protect Mr. Sakharov's future. Probably the most potent would be the scientific exchanges. These programs are highly valued by the Soviet government, both because of the direct payoffs in new knowledge and technology and because they are a means of bolstering and morale of the Soviet Union's scientists, who are well aware that they are cut off from the international sharing of ideas that occurs in most of the rest of the scientific world.

So far, the response of the American scientific community has been encouraging. The National Academy of Sciences, normally the most cautious and slowly moved of bureaucracies, issued a statement within 24 hours of Izvestia's latest charge. "This blatantly punitive act . . . can only be regarded as a challenge to further cooperation," wrote Academy President Philip Handler. "I find it difficult to imagine scientific exchange continuing in the spirit we had created heretofore." In a message to Soviet Ambassador. Anatoliy Dobrynin, the American Association for the Advancement of Science warned that the actions against Mr. Sakharov will "unavoidably undermine" scientific cooperation between the United States and the Soviet Union. The American Society of Civil Engineers had already halted all exchanges following the invasion of Afghanistan.

The leaders of the Soviet government must understand that they cannot move with impunity against a man who, in the words of a Russian writer, "incarnates the conscience of Russia."