Moscow — The Soviets are getting worried that they, and not the Americans, may emerge the prime superpower loser from the latest Mideast war.
In a conversation with the Monitor July 9 - on the heels of a Kremlin note to President Reagan cautioning him against sending United States troops to Lebanon - a ranking Soviet official commented:
''The US can win a lot of points if it takes the Palestinians out'' of entrapment in Beirut.
He said he thought it conceivable that the Americans would achieve this. From the standpoint of US interests, ''So far the (Reagan) administration seems to be acting in a basically wise manner,'' he said. After a pause, he added with a half smile the word ''unfortunately.''
If the negotiating mission of US envoy Philip C. Habib ultimately succeeds, and if a limited US troop contingent helps to implement the accord smoothly, ''The Palestinians will say thank you. . . . The Lebanese will say thank you,'' the official said. ''The influence of radical Arabs (after the conflict) will decrease. The moderates will gain. . . .''
And if a reconstructed Lebanon pens a peace with Israel, ''This will increase the role of the Camp David process'' rooted in the US-sponsored treaty between Israel and Egypt.
''Accordingly, the influence of the Soviet Union (in the Mideast) could decrease,'' he said.
The official made clear that a number of Mideast ''ifs'' remain. Yet his survey of conceivable American gains contrasted with earlier remarks on the crisis in an interview with the Monitor about a week after Israel's June 6 invasion of Lebanon. At that point, this official deemed it most likely that widespread Arab bitterness over the Israeli blitz would rub off on the Americans , that anti-American Arabs would emerge strengthened, and that Washington would thus lose regional influence.
He and other officials interviewed by the Monitor stressed that ''realism'' has been the watchword so far in charting Soviet response to the Lebanon crisis.
It was against this background, they suggested, that the July 7 letter from President Leonid Brezhnev to Mr. Reagan should be seen. The note said that if the Americans did send troops to Beirut, ''the Soviet Union would build its policy with due consideration of this fact.'' But the message was worded less explicitly and ominously than a Brezhnev note at the height of superpower tension in the 1973 Mideast war. A July 11 Pravda article cautioned France against joining a Beirut troop force, because this ''could boomerang against the prestige of French policy.''
By ''realism,'' Soviet officials mean accepting that, no matter how bitterly they and allied Arabs resent Israel's invasion, practical responses are limited. The Palestinians are physically in no position to fight their way out of Israeli entrapment, one official noted. Others said that neither the Soviets nor the Syrians saw much to gain from a full-scale showdown between Israel and Syria.
These considerations, officials say, explain the generally restrained tone of Soviet policy statements so far. As one Soviet source remarked straightforwardly , ''If there are limits of realistic possibilities, the less words, the better.'' He added that, with this in mind, he personally questioned the value of the Brezhnev letter on the US troop issue.
Yet he also said the letter was a move that the Soviet Union, as a superpower , felt necessary, and that further, similar actions seemed likely.
It is this ''superpower factor'' that most concerns foreign diplomats here. The worry is that the longer the Lebanon stalemate and its chorus of artillery duels persist, the greater the risk that Kremlin emotion - and Kremlin concern over potential US political gains - may prod a reassessment of the current policy line.
Should the present line prevail, officials suggest, the next forum for superpower jousting in the Mideast is likely to be diplomatic, as both Washington and Moscow maneuver for influence in the Arab world once the guns in Lebanon are silent.