IN ITS OPENING WEEKS, the Reagan administration has denounced the Soviet Union almost daily, drawn a line against communism in El Salvador and moved to sharply increase the defense budget. The reaction from Moscow has been curious: a proposal from Leonid Brezhnev for summit talks, disarmament initiatives, and suggestions that lucrative trade with the United States might be improved.

Just what are those shifty old men in the Kremlin up to? Logically, they should be fulminating and warning of a military buildup of their own. This pretense of reasonableness seems distinctly sinister. How dare they do any purring when we are in full bristle!

I don't have the definitive explanation for the Soviets' refusal to cooperate. That must await publication of Brezhnev's memoirs. But I do have a theory: Discomfiting as it may be to Reagan policymakers, Moscow actually finds all the nastiness from Washington reassuring in some ways, even convenient, a return to the familiar frostiness that has marked U.S.-Soviet relations for most of the last 35 years.

"Controlled hostility," as a Russian once described this condition to me, is a good deal easier to manage than detente. Or, putting the issue another way, the Soviets, as a rule, are not subtle. Ambiquities make them twitchy. And one thing is for sure now: The Kremlin knows where Ronald Reagan stands.

For the sake of comparison, cast back to the early weeks of the Jimmy Carter era. Then Moscow faced a new president who repeatedly declared his commitment to improved relations with the Soviets -- a quick strategic arms accord topped the list.

But Carter proceeded to baffle the Kremlin by embracing the cause of Andrei Sakharov and other leading Soviet dissidents. The confused Politburo assailed Washington's egregious intervention in their "internal affairs." Carter, the Russians whined, was "unreliable."

Next, Secretary of State Cyrus Vance arrived in Moscow with a sweeping proposition for arms reductions that would have meant deep cuts in the Soviet arsenal instead of the nuclear cap envisioned in the 1976 Vladivostok agreement between Brezhnev and President Ford. Once again the Soviets were flabbergasted, caught off balance and incensed. The American ideas were a "cheap and shady maneuver," declared Foreign Minister Andrei Gromyko.

Noting Moscow's mellowness now in contrast to four years ago, Marshal Shulman, a top adviser on Soviet matters in the Carter administration, wryly observed the other day that "relations are not better than they were, but they are simpler." Consistency turns out to be a virtue welcomed in Moscow, as it would be anywhere. "In dealing with the Soviets," Schulman said, "there is less danger in a situation that is consistent, even if it is bad."

So if that reasoning is correct -- and I think it is -- the Kremlin finds the clarity of Reagan's rhetoric, ironically, to its liking.

Moreover, with the United States at figurative battle stations, Moscow can afford to take the high road, as Brezhnev did in his opening address to the Communist Party congress last month. The Soviet leader doubtless got brownie points somewhere -- among the left in Europe, for instance -- with his lofty proposals for summit talks in the vague future. Meanwhile he left himself and his colleagues time for dealing with pressing problems like the crises in Poland and Afghanistan. Both of these seemingly intractable situations have to be eased anyway before significant headway can be made with the United States. And from the Soviet perspective, it is a whole lot easier to take decisive action with Washington and Moscow already at loggerheads than it would be if the Soviets were seriously trying to win favor with us.

Suppose, for instance, that we were living in another time. Detente is in full bloom, trade is clipping along, a host of disarmament negotiations are moving ahead and then -- suddenly -- the Poles revolt. Would detente stop the Soviets from intervening? Not if they absolutely felt they had to. America thus betrayed might strike back violently. To hear Reagan declaim it now, we already expect the worst from the Soviets, including an invasion of Poland. Anything less would come as a pleasant surprise.

On other fronts, the Reagan hard line provides Moscow with an excuse, if it wants one, for easing its contentious activities in the Third World. With all its internal economic difficulties on top of the Poland-Afganistan headaches (never mind chaos over the border in Iran and the continued enmity of China), the Soviets are already very busy. While Washington is in its truculent phase, ready to block Soviet-supported Cuban forays like the one in El Salvador, Moscow can go into a consolidation period. The Soviets have enough to do in Angola, Ethiopia, South Yemen, Vietnam and Cambodia without hustling new commitments.

Lenin, after all, put the maxim for communist advances plainly: two steps forward and one step back.

Well, the natural next question is: Aren't the Soviets worried about the prospect of an unrestricted American military buildup? Undoubtedly they would prefer, given some druthers, that it did not happen. The Soviets are spending a much bigger share of their GNP on defense than we are and the notion that they might have to shell out more in a hellbent new arms race can hardly be attractive. But that aside, I suspect, the Kremlin never thought we were as weak as we did anyway.

Soviet gains in Indochina, Africa and Central America have, by and large, been the result of self-imposed American restraints -- the withdrawal from Vietnam, the ban of aid to insurgents in Angola, the disavowal of Somoza in Nicaraugua -- not because the Soviets had more or better weapons than we did.

In a world with a delicate balance of nuclear terror, the United States is only as strong as it thinks it is. We can spend a trillion dollars on defense in the next five years but if we conclude that the Soviets have spent a trillion and one and are therefore stronger, than all the effort is nullified. That, at least, is the message the present American insistence on our vulnerability probably conveys to Moscow. It is crazy when you think about it very long that the United States has amassed the most formidable strategic arsenal in history, capable of destroying the world in hours, and feels threatened. But it does.

Throughout the detente era, the prevailing American view was that we generously offered the hand of friendship and the Soviets bit it at every turn; that we tendered detente to an adversary that needed and got more from cooperation than we did.

In fact, that assessment is wrong. If Moscow were so reliant on good relations with the United States, then its trade with Europe would not be growing, its military strength would not be increasing and its reach into the Third World would not be so long. The purpose of detente was to rein in the Soviets, shackle them with agreements that would give us leverage to use against them if they misbehaved.Not surprisingly, they may well prefer to be unleased. Ronald Reagan clearly prefers that for the United States.

Finally, some Soviet strategists are probably arguing that Reagan's opening gambits won't last. Ronald Reagan is Leonid Brezhnev's fifth president. Each one had a different style.Yet each came to the eventual conclusion that bargaining with the Soviets on the crucial, core issue of strategic arms was unavoidable.

Lyndon Johnson built up the U.S. commitment to Vietnam and went to Glassboro for tentatvie SALT talks with Premier Alexei Kosygin. Richard Nixon undertook the biggest bombing campaign in history against Soviet-allied North Vietnam and sipped champagne in Moscow, SALT accord in hand. Gerald Ford quickly reached agreement on a framework for SALT II but couldn't quite get it signed. Jimmy Carter got it signed but couldn't get it ratified.

Moscow's view is that America's struggle over how to deal with the Soviets is more with itself than with the Soviets.

If all this makes it sound like the Soviets have it knocked no matter what we do, they certainly do not. A resurgent United States intent on a military binge obviously poses more of a threat to the Kremlin than a Vietnam-syndrome America reluctant to use its own power. Still, my theory holds that the Soviets see a silver lining to all the surliness from the Potomac, which is why they are being so nice about it.

If coming on strong to the Soviets makes President Reagan, his administration and the American people feel confident again, then perhaps (the Kremlin hopes), the United States will be better prepared to accept the uncontestable realities of Soviet power. Because the Kremlin knows -- and I bet Americans do, too -- that no matter how hard Ronald Reagan huffs and puffs, the Soviet house will not fall down.

It would have to be bombed away and neither side, yet, seems ready for that.