The scenario of events now predicted for the completion of a new strategic arms agreement with the Soviet Union raises the prospect of a fascinating and unprecedented political gamble by Leonid Brezhnev, the leader of the Soviet state and Communist Party.

Once a new SALT agreement is signed, American attention will rightly be focused on the Senate, where debate on the treaty is certain to be tense and the outcome is likely to be close. But a thought or two should be reserved for Brezhnev's dilemma-one he may not fully appreciate himself.

It is this: By Soviet standards, Brezhnev's participation in a summit meeting on American soil at which new SALT agreements are signed will represent an unreserved personal commitment. Soviet news media can be expected to hail the summit as a great victory for Brezhnev's "piece policy," as it is called at home. The new SALT agreements will appear in Moscow to be Brezhnev's personal SALT agreements.

Willy-nilly, then, the summit meeting will make Brezhnev's political standing at home hostage to the deliberations of the U.S. Senate, for the Senate will be able to alter or reject the agreements that Brezhnev so dramatically will embrace.

Why would the Soviet leader take such a risk? In large measure, probably, because Soviet leaders truly do not understand the polycentric nature of American politics. They are used to a system in which the leader speaks for the entire government-a monocentric system that does not allow for the open clash of views, let alone the veto of the leader's own decisions by forces beyound his control.

Americans who have discussed the Senate's role in SALT ARE REPEATEDLY STRUCK BY SOVIET UNWILLINGNESS TO ACCEPT THE FACT THAT 34 SENATORS MIGHT INDEED OVERRULE THE PRESIDENT ON THIS ISSUE. "SURELY," A SENIOR SOVIET OFFICIAL SAID IN A PRIVATE CONVERSATION IN WASHINGTON THE OTHER DAY, "THE FACT THAT THE PRESIDENT SIGNS THE TREATY WILL MEAN THAT THE SENATE MUST APPROVE IT-THIS IS AN ISSUE OF STATE."

IN MOSCOW LAST MONTH, THE SOVIET PREMIER, ALEXEI KOSYGIN, LECTURED A DELEGATION OF 12 SENATORS ON THE INEVITABILITY OF THEIR SUPPORT FOR THHE SALT agreements. "You know that we know what the U.S. Senate is," Kosygin announced coldly. He then proceeded to demonstrate to the senator's satisfaction-judging by conversations with 10 of the 12-that he had no idea what the Senate really is like.

In fact, there are probably 25 senators who would vote against the SALT II agreements even if they provided for unilateral Soviet disarmament. It will only take 34 senators to block the agreements.

Hamilton Jordan, President Carter's principal political tactician, is so uncertain of the outcome in the Senate that he is still toying with the idea of Submitting SALT to both houses of Congress as an executive agreement, requiring only 51 percent approval instead of two-thirds. Such a move is unlikely because it would enrage Senate leaders, beginning with Majority Leader Robert C. Byrd (D-W.Va.), but the fact that Jordan is still considering it is a demonstration of the administration's lack of confidence.

It could be argued that Brezhnev's personal commitment to SALT II could become a telling administration argument in the White House's efforts to win Senate approval for the agreements. But it could be unwise for the administration to try that tack.

Opponents of SALT could be expected to counter that it is no business of an American administration to try to bully the Senate with agruments about the fate of foreign leaders. Senators are usually sensitive to any infringement of what they see as their prerogatives, and the argument that the Senate has an obligation to preserve Leonid Brezhnev's power in Russia might well be interpreted as a flagrant intrusion into the Senate's rightful domain.

Even if the administration is successful in lobbying for SALT II and the necessary two-thirds of the Senate decides to support it, the SALT debate will inevitably include rhetoric that the Soviets-and Brezhnev personally-will find extremely offensive. Omar Torrijos, Panama's strongman, has told recent visitors he destroyed a dozen transistor radios during the Senate debate o the Panama Canal treatties, which were simultaneously broadcast in Panama in Spanish. The trasistors broke, according to Torrijos, when he hurled them against the wall in anger.

The Soviets will not broadcast the debate in Russian on their radio stations, but they won't have to. The Russian language services of the Voice of America, British Broadcasting Corporation and West German Radio will inevitably carry extensive accounts of the Senate's debate to the Soviet public. Soviet leaders-whose sensitivity about their own prestige cannot be exaggerated-will inevitably find themselves vividly condemned before their own citizens in a novel way.

The upshot of all this cannot be foreseen. But the developing situation is a powerful reminder of the fundamental differences between the two superpowers that have made it so difficult for them to find a basis for mutual understanding and cooperation.