Just when Americans thought they had sorted out the big two of Russian dissidents, one of them--Andrei Sakharov, the Nobel Prize-winning physicist whom the Kremlin banished to internal exile three years ago--has confounded a good part of his following in the West.

He has done so, moreover, in a way likely to narrow the perceived difference between him and the other celebrity exile, Alexander Solzhenitsyn, the Nobel Prize-winning writer now living in Vermont.

Sakharov has been the liberals' favorite: a man of great professional achievement and personal courage who, from a dangerous and dramatic perch within the Soviet Union, has upheld universal values of human rights, d,etente and peace. By contrast, Solzhenitsyn wore out much of his Western welcome by turning out in exile to be not just a great writer and a brave man but an old-fashioned Russian nationalist with a scarcely concealed contempt for the Western public's slowness to embrace his passionate, moralistic brand of anti-communism and anti- Sovietism.

Now comes Sakharov in Foreign Affairs magazine. He starts out in an unexceptionably liberal vein by affirming, in his article's title, "The Danger of Thermonuclear War." He makes his case in the deeply humanistic terms that have earned him his broad Western appeal.

His anti-nuclear war logic then leads him, also unexceptionably, to a call for conventional weapons parity--which he believes the West does not currently possess--in order to diminish the reliance that Western strategy now places on early recourse to nuclear arms. For this, he says, "certai limited economic sacrifices" are required, plus "an understanding of the seriousness of the situation . . ."

It is the next step in his anti-war logic that most fazes his admirers. Sakharov distinguishes between the "arms race" and the danger of war. He believes the latter arises not, as many believe, simply from the accumulation of arms but, as Ronald Reagan believes, from the Soviet Union's advantage in certain measures of overall strategic power and even in certain local theater or regional contexts. Therefore, he suggests, the West must build up, including perhaps the MX.

"Of course I realize that in attempting not to lag behind a potential enemy in any way, we condemn ourselves to an arms race that is tragic in a world with so many critical problems admitting of no delay,'' Sakharov says. "But the main danger is slipping into an all-out nuclear war. If the probability of such an outcome could be reduced at the cost of another ten or fifteen years of the arms race, then perhaps that price must be paid while, at the same time, diplomatic, economic, ideological, political, cultural and social efforts are made to prevent a war."

Through his text he sprinkles some remarkable--for a Soviet--observations on the Soviet regime: one should not rely "on one's potential enemy to be perfectly sensible"; nor should one proceed "from an a priori assumption of any special peace-loving nature in the socialist countries due to their supposed progressiveness or the horrors and losses they have experienced in war"; "the general trend of events since 1945 . . . is nothing but Soviet expansion on a world scale"; Soviet power "has today assumed proportions dangerously harmful to international equilibrium."

You do not have to accept Sakharov's whole argument--his reading of the current military balance, for instance, or his readiness to countenance another 15 years of unconstrained arms building--in order to acknowledge the importance of his article in at least two dimensions.

Politically, it is something of a seismic event. Reagan could not have asked for a more impressive witness-- the single authoritative independent voice in the Soviet Union--for major aspects of his strategic policy.

As for Sakharov, he could have been under no illusions as to how his article would be read in the Kremlin. Currently he lives a life of internal exile, extreme police harassment and denial of vital medical care--on a Soviet scale, only a medium-bad condition. The unprecedented attack on his article now by four members of the Soviet Academy of Sciences suggests he may finally be losing the Academy tie that has constituted his single visible shield against all-out official vengeance.

Neither Sakharov's service to what he believes nor his ordeal is at an end.