THE ARREST LAST month in Rome of a Bulgarian government airline official on charges of direct involvement in the 1981 attempt to assassinate the pope has introduced a poisonous new element into the Western relationship with the Soviet Union -- an element that could also further divide the NATO alliance.

This arrest has suddenly made respectable suspicions that had previously seemed irresponsible: that the Bulgarian regime had manipulated the Turkish terrorists who attempted to kill the pope; and that behind the Bulgarian government, the most closely controlled Soviet satellite, necessarily stood the Soviet KGB, its chairman in 1981 -- Yuri Andropov -- and the Brezhnev Politburo.

Now it has become plausible to offer a new conjecture to explain the apparently motiveless attack on the pope: that the Brezhnev regime in its final years had embraced not only terrorism but assassination, and had sought through intermediaries to remove the man it identified as one of the main causes of the deterioration of the Soviet position in Poland. (In May 1981, when the assassination attempt occurred, the Soviets' problems in Poland had reached their most desperate state.)

This hypothesis, if accepted, would require a basic revision of assumptions long held in the West about the Soviet regime. Until now it has been commonly believed that the Politburo is fundamentally cautious, reluctant to accept serious risks, disinclined to take "adventurist" actions, and particularly averse to involvement in the assassination of major Western public figures -- not on moral grounds, but because assassinations might have drastic and unpredictable consequences.

If these assumptions were incorrect, then what criteria are the Soviets now using to judge the risks and benefits of different kinds of international behavior? Has the threshold of acceptable risk now been raised? Do the Soviets now calculate that the deterioration of Western morale and the growth of Soviet military power have gone far enough to impart a margin of safety to actions previously considered too dangerous?

Before reaching such conclusions, we must address three questions about the evidence:

First: Is the Italian evidence against the Bulgarians as solid as the press leaks have made it out to be? Until we can pass beyond the leak stage, until the world can hear the testimony of the would-be Turkish assassin, Mehmet Ali Agca, (presumably at the trial of the Bulgarian airline official, Sergei Ivanov Antonov), and until some independent corroboration of Agca's statement is provided, this point will not be nailed down. We must note, however, that Italian Premier Amintori Fanfani, and -- just last week -- the socialist Italian Defense Minister Lelio Lagorio have now thrown the prestige of the Italian government behind the allegations.

Second: Could the Soviets be such bunglers as to have allowed the Bulgarians, so closely identified with themselves, such a direct and visible role in assisting Agca?

Third: Is it credible that the Soviets thought they could actually solve their Polish problem by eliminating the Polish pope? Or that they could get such significant benefit from his removal that assassinating him was worth all the attendant risks?

It has been suggested that the Soviets' intention was to dampen the fires in Poland; that the conduct of the Polish church since the assassination attempt shows that it has indeed been intimidated; and that the Soviet leaders also wished through this action to intimidate the West generally, and therefore were willing to allow unprovable suspicions to be direcred at them.

This explanation is not convincing. The Soviets and the Jaruzelski regime have indeed wished to intimidate the Polish bishops into accommodation, but the relative passivity of the Polish church in recent months is a reaction to the regime's success in supressing Solidarity, not to the attack on the pope.

And it is difficult to imagine that the Soviets could put so high a value on the intimidation of the West as to intend a Soviet role to be widely suspected. They cannot yet have so low an opinion of the West as to confidently expect the adverse consequences of such suspicions to be trivial.

If this explanation is not accepted, then the questions I have posed about the adequacy of the Soviet motive and the clumsiness implicit in the direct use of Bulgarians have not yet been answered. Nevertheless, if the Italian accusations against the Bulgarians stand up, the chain of circumstantial evidence implicating the Soviet leadership is grave indeed.

The Turkish killer Agca is believed to have visited Bulgaria for some time in 1980 after he had murdered a Turkish editor and escaped from a Turkish prison. It seems unlikely that the Bulgarian leadership -- and the Soviets -- were unaware of this. If the Bulgarian airline official Antonov, and the two Bulgarian embassy employees alleged to have been involved with him in the plot to kill him Pope, are indeed guilty, then it is difficult to believe that the Bulgarian goverment was not deeply involved. It is equally difficult to imagine a private motive for three members of the Bulgarian intelligence service to organize this crime. And it is even harder to imagine that the Bulgarian leadership could remain ignorant of a matter of this gravity.

It is hardest of all to envision the Bulgarians plotting the pope's assassination without Moscow being fully aware of what was happening. Bulgaria has always had more intimate ties with the U.S.S.R. (and, indeed, with czarist Russia before it) than any other country. Its language and culture are closer to Russia than those of any other East Eurpean state. It is the most willing Soviet vassal state; the Soviets do not need to station troops in Bulgaria to hold it down. Bulgarian foreign policy is a flexible and reliable instrument of Soviet foreign policy, and the Bulgarian intelligence service, massively penetrated by the KGB, is its most obedient subsidiary.

Finally, if the Soviet KGB was involved, it is not plausible that its chairman, Andropov, Brezhnev and other Kremlin leaders, were not also involved. For the Soviets do not delegate authority on matters this momentous. If any Soviets were involved at all, it is not credible that an issue with this explosive potential was not approved at the highest level.

To many in the West, the consequences of concluding that the Soviets took part in the plot to kill the pope are so appalling that the matter will simply not bear thinking about. In view of the widespread fear of war and desire for better relations with the Soviet Union, there is a strong temptation to push the question away. A few will accept the Bulgarian contention that the Bulgarian official was framed; others will prefer to believe that he was acting alone; still others may concede that the Bulgarian government may have been involved, but will contend that this does not necessarily prove Soviet involvement. (The issue so far, of course, is not one of proof, but of probabilities.) Many more will simply prefer to leave the matter unconsidered.

The West's general reluctance to confront the possibility of Soviet and Bulgarian complicity in this assassination is not surprising. For if the charges were known with confidence to be true, if it were once accepted that the Soviet leaders had indeed adopted a policy of murdering Western leaders, what Western policy toward the Soviet Union would be commensurate? What dealings with the Soviet Union would then be appropriate? What place would the whole panoply of present Western interactions with the U.S.S.R. -- from arms control to commercial relations -- have in a universe in which it was known that this was Soviet policy? And if the Soviets would do this, what else would they do?

Since the implications here are intolerably dangerous, much better that the hypothesis not be true. It shouldn't be surprising, therefore, if many in the West have an unspoken conviction that the stability of the world-as- it-is demands that the Soviets be innocent. This conviction has an unfortunate effect on the evalutation of evidence.

This response has a certain similarity to the Western reaction to the issue raised by the Soviet production of "Yellow Rain" in Indochina. Soviet manufacture of mycotoxins and transfer of them to the Vietnamese for use in punitive operations, by violating one arms control agreement, has raised grave questions about the Soviet attitude toward all such agreements -- questions that are generally put to one side by the Western public. There is a powerful tendency in many quarters to cling to ambiguity and obscurity as protective shields to avoid the necessity of contemplating the totality of Soviet behavior and factoring this into Western policy.

To be sure, the issues raised by the possibility of Soviet complicity in the attempt on the pople's life are far more serious than those involved in "Yellow Rain," and the evidence is still less conclusive. But demonstrated Soviet guilt in the lesser case should at least predispose us now to pay attention.

On the other hand, others in the West, particularly in this U.S. administration, may see the arrest of the Bulgarian merely as confirmation of their deeply held convictions about the Soviet regime. These people have never accepted the prevailing view of western Sovietologists that the Soviets are inhibited from taking adventurist actions. But if those who expect the worst from the Soviets are eventually impelled to respond to them unilaterally, and more confrontationally than before, this could make it even more difficult to maintain a consensus in the United States -- let alone in the West -- to effectively counter Soviet policy. It is possible that one consequence of the attempted papal assassination may be to introduce a new factor that will profoundly divide Western societies and paralyze Western policy.

Nevertheless, this matter cannot be allowed to remain indefinitely pigeonholed by ambiguity. It is unfortunately possible that conclusive evidence may never be available. But conclusions must in due course be drawn, one way or the other, about the probability of Soviet guilt. For now, western editorial boards have a duty to their societies to give this matter a priority which many of them will find unpleasant. The United States government will eventually have to take some official position on the matter, and should surely begin a process of consultation with our NATO allies, Japan, and other friends to search for an appropriate Western response in the worst contingency.