EVEN AS Ronald Reagan held his first press conference as president-elect and affirmed the tough anti-communist line on which he was elected, Soviet spokesmen, from the new prime minister, Nikolai Tikhonov, down through the mood-music makers, were extending him an ostentatious welcome. At the same time, his ideological soul mate, ex-president Richard Nixon, was being lionized at the Soviet national day party in Washington.

It seems like a contradiction, but it isn't. Moscow has always been ready, though it can grump about it, to play the bounces in American politics and to offer an atmospheric "honeymoon" to a White House newcomer. Moreover, much as the Kremlin may fulminate over American "cold warriors," its own ideology conditions it to expect the breed to come to power, and experience teaches it that these are often more consistent and businesslike and better able to deliver on policy promises than others.

What is the real Soviet attitude? We can guess that Jimmy Carter's zigzagging, his human rights kick and his weakness at home finally exasperated the Soviets and made them pine for a renewal of the detente policy represented by former president Nixon and his fellow guest at the embassy reception, Henry Kissinger. Detente in this sense means: a nuclear dialogue and a crack at access to Western credits and technology, within the context of a continuing global struggle conducted by certain rough rules (acceptance of the other fellow's domestic legitimacy, primacy within one's sphere of influence, a right to aid one's friends outside that sphere). You could call it hardball, but played on a diamond, not a sandlot.

The question for Moscow right now is whether Mr. Reagan intends to play that sort of tough but familiar and generally manageable game. Or does he really believe what he has been saying (with the latter-day Nixon and Kissinger trailing in assent) about the need to revise the old terms of detente and to gain an upper hand over Moscow? The answer will affect the Soviet struggle already in train over the succession to the septuagenarians who still rule in the Kremlin. It will also affect the Soviet budget debate -- the next five-year plan is due to be written in early 1981.

The Reagan choice is also bound to affect Soviet policy, especially in the two places, Afghanistan and Poland, where Soviet military action is either now taking place or being implicitly threatened. This is the crux. Four years ago when Jimmy Carter arrived in the White House, actual or potentially imminent Soviet military action, inside or outside the bloc, was simply not part of the picture. Today it is in the center of the picture.

In his first statement not intended for a campaign audience, Mr. Reagan said that not just arms control but Soviet "policies of aggression" must be put on the table. Does this not cover Poland, which is in the Soviet block and thereby ostensibly prey to Soviet intervention under the Nixon-Kissinger detente rules, and Afghanistan, which might be considered partly in and partly out of the bloc? Mr. Reagan does not seem to have decided whether "linkage," which he supports, ties arms control to the two places currently most exposed to Soviet wrath. It appears that he wants Moscow to believe that it does, but he does not want to lock himself in.

It is probably wise to start out in this ambiguous way and to give himself a little room to maneuver. Meanwhile, Americans who voted for Jimmy Carter have something to think over. It is different now that Ronald Reagan is about to be president. His image as a tough anti-Soviet politician could turn out to have positive value if it helps give the Soviet juggernaut pause. The American interest self-evidently requires that he avoid rash and gratuitous provocations and that he seize what openings for improving Soviet-American relations may exist. But it would also serve the American interest for the Soviet Union to understand that Ronald Reagan has a strong mandate to counter and check the widely perceived flow of the Soviet military tide. You don't have to have voted for Ronald Reagan to hope that he succeeds.