With the end of the Brezhnev era, the United States and the Soviet Union are at one of the lowest points in their turbulent rivalry, headed down a bleak road of tension and mutual suspicion.

The basic antagonism between the two superpowers has so intensified since the collapse of American-Soviet detente, and the advent of the Reagan administration, that it is not susceptible to fundamental change.

There is, nevertheless, an outside chance for using the transition in the Soviet leadership to moderate the tension between the two nations, many specialists on both sides agree, if the opportunity is seized early in the turnover of Soviet power. But the formidable obstacle is that each power sees itself as the aggrieved party, waiting for the adversary to take the initiative by altering its stand.

President Reagan, in his message of condolence on the death of Soviet President Leonid I. Brezhnev, has made what his administration sees as an overture to the new Soviet leadership for "expanding the areas where our two nations can cooperate . . . "

It is improbable, however, that the new leaders will construe that message alone as a significant gesture signifying any change of American intent, especially in the absence of Reagan at the head of the American delegation to Brezhnev's funeral.

Experts on the Soviet Union generally caution against overreading the significance of personalities in shaping Kremlin policy, especially at the outset of any new regime. There is much greater consistency, and predictability, in Soviet policy than most nonspecialists recognize.

At the same time, over the long term, there have been marked changes in the style and tone of Soviet performance as the most durable leaders emerged, from Lenin to Stalin, from Stalin to Khrushchev, and from Khrushchev to Brezhnev. The course of American-Soviet relations essentially changes only marginally. But it can be in these marginal shifts that critical choices are made on confrontation or accommodation.

In this instance, Brezhnev's death comes at an acute stage of acrimony in American-Soviet history, which is likely to leave its mark on his successors.

Brezhnev, in his last major public address, on Oct. 27 before the leadership of the entire Soviet military structure, portrayed himself as a man who had gone the last mile with the United States in pursuit of his goal of detente, only to be rebuffed by the Reagan administration.

"Two lines now clash in world politics," Brezhnev said: "The line of the U.S.A. . . for deepening tension and aggravating the situation to a maximum," in contrast to "our line . . . for detente and strengthening international security."

At no point in that Brezhnev formulation, or earlier ones, was there any acknowledgment that the American and Soviet interpretations of detente have been profoundly at odds about what each nation was committed to deliver.

This huge gulf between Moscow and Washington is of far greater significance for the two nations' joint fate than whether Yuri Andropov or Konstantin Chernenko, or someone else, emerges as Brezhnev's successor.

Every American president and secretary of state, in remarkably similar words, has called on the Soviet Union for a basic change in its pattern of international conduct -- with no prospect whatever that it will occur. Secretary of State George P. Shultz did so in a recent interview, saying:

"If you change your behavior, Mr. Soviet Union, you can get a good response from us. But in the meantime you have an adversary that is strong and is determined and can take care of itself."

No nation, however, is amenable to "changing its behavior." What has brought the United States to a frigid impasse with the Soviet Union is the Reagan administration's exceptional determination to try to compel a change in Soviet global behavior: By confronting it with superior American strength, even at the risk of an open-ended arms race, or even a confrontation.

On its face, this would appear to put the two superpowers on an inevitable collision course. It is particularly notable, therefore, that in spite of the heightened bristling on both sides since the Reagan administration took office, the two nations have avoided a head-on confrontation.

There has been an abundance of regional conflicts that could have spiraled into an American-Soviet crisis: in the Persian Gulf, in the Falklands war, in the Lebanon war, in Central America and elsewhere.

Perhaps it was only luck that the two superpowers have avoided being drawn into a collision. But the record also suggests that despite their bitter antagonism, both superpowers recognize the danger of stumbling to the brink by miscalculation.

However, what the two nations are more likely to miscalculate at a time of high tension between them, the historical record suggests, are fleeting opportunities to curb tension in limited areas where their interests could converge.

This may possibly be such a "window of opportunity," according to many specialists in and out of the government -- the most critical choice for the Reagan administration in its entire East-West policy.

The decision-making level of the government, however, is now weaker in experience in dealing with the Soviet Union than it has been at any point since World War II. It is also not venturesome in probing for new departures in policy. Halfway through the life of this administration, President Reagan is not inclined to make any shift of course in his Soviet policy, unless his advisers can produce compelling reasons for doing so.

Through a combination of circumstances, there are no seasoned Soviet experts at high levels in the White House or in the State or Defense departments who can readily draw potential initiatives out of their direct knowledge of the past. This automatically heightens the influence of specialists who have been reinforcing the Reagan administration's proclivities for holding a militant course toward the Soviet Union.

That advice from the overwhelmingly predominant hawkish specialists is that the Soviet Union is in "acute crisis": in its economy, in its political structure, in its immediate sphere of control in Eastern Europe and in the crushing cost of its global reach.

By maintaining maximum pressure upon the Soviet Union, these advisers insist, the United States either can force it to turn inward on itself, and thereby reduce its capacity to expand its global influence, or require it to reform and liberalize its rule.

The opposing school of advice -- very weak in its influence within the administration -- agrees that the Soviet Union has crippling, long-term, systemic disabilities. But these specialists maintain that the Soviet Union also has formidable durability, which has been demonstrated through the Brezhnev years. It will not, they maintain, readily yield to any amount of external pressure, but on the contrary, is far more likely to magnify the danger to the United States if it sees itself cornered.

What the United States should have done earlier, and now has a fleeting opportunity to do, these specialists maintain, is to hold out to Brezhnev's successors a clearer, alternative course: a more tangible mixture of incentives to temper its militancy, to reach accommodations with the United States.

Even if the overture is rebuffed, the advocates of this course argue, there would be no loss in exploring "a window" that may not open again in the life of the Reagan administration.

In his press conference last night, Reagan gave no indication that he is tempted to head in that direction. By firmly reiterating his conviction that "peace is a product of strength," he took the position that he has left the existing window open wide enough for the Soviet Union to squeeze through if it will only make the effort.

The administration's message to the new, evolving Soviet leadership, therefore, at least at this stage, matches its stand in domestic policy to the extent it can be sustained. It is that the United States will "stay the course."