Is that Ronald Reagan, the well- known hard-line anticommunist, saying nice things about the Russians these days? He's "welcomed" their new offer of a 50 percent cut in offensive strategic arms and implicitly endowed them with readiness for "tough but fair negotiating."

It sure was Reagan, and it will alternately please those who feared and dismay those who hoped he would never swerve from his more characteristic stance. But almost certainly it does not mean that the president has altered his basic views or that he has come under the spell of either of the two people sometimes credited with exercising, in their respective ways, a special influence on him.

One of these is Mikhail Gorbachev, widely seen as a master manipulator of Western opinion, and the other is Nancy Reagan, whom some insiders often depict as determined to ensure that her husband goes down in history as a champion of peace.

No, something else may be happening. In his first term, Reagan, who had taken on the job of restoring American power, necessarily defined the American interest in terms of what he thought was desirable in relations with the Soviet Union. Now, having in his view accomplished his mission (and few will quibble), he is moving on to think in terms of what is possible in relations with the Soviet Union. Not everything desirable is possible. Not everything possible is desirable. But Reagan is zeroing in on that vital intersection.

Skeptics observe that Reagan, like Gorbachev, may just be playing a public relations game. Liberals say this out of distrust of the president and conservatives out of a distrust of public opinion, which they regard as so insidious and alluring that not even their tough-minded chief may be immune to its wiles. In this evident spirit, by the way, Dean Rusk's 25-year-old warning about whether a president "can wisely undertake the burdens and hazards of personal diplomacy at the summit" is circulating in administration circles -- the 1960 Foreign Affairs article in which the Kennedy-Johnson secretary of state suggested that "public opinion is moved by desperate hope and the fascinations of the spectacular. . . ."

For what it's worth, I think Reagan, far from playing a game, is serious. Confident might be a better word. Notwithstanding the occasional reflexive lament that the United States is "still behind" Moscow, he seems confident of the success of his first-term military buildup; surely he sees the advantage of negotiating before Congress chews further into the rate of future military growth. He is confident in his analysis of the Soviet Union as a power needing economic respite. People who know him insist he is confident of his personal prowess as a negotiatr.

For what more it's worth, I think Gorbachev is no less serious. Certainly, the personal stakes are higher for him. By reversing his erstwhile patron Yuri Andropov's we-can't-deal-with- Reagan line, new-boy Gorbachev took a political risk he must justify -- an agreement might be the best way -- by the Communist Party congress next February. Perhaps that helps explain why he has just broken with the familiar Soviet pattern of leaving it to the West to make a proposal, which both sides then whittle down, and instead has put a Soviet proposal on the table at Geneva.

Some Westerners scowl that Gorbachev's direct approach to Western opinion lets him peddle snake oil. But the year's record of Soviet movement shows, I think, that Western opinion is no pushover for a Kremlin leader and that Gorbachev's appeal to it is forcing him to make Soviet positions more responsive -- not less -- to American needs.

Still, a summit success is far from assured. A large gap remains on the crucial details of strategic offense and on the whole concept of strategic defense. Even as President Reagan was broadly welcoming the Soviet offensive-arms proposal, Richard Perle in the Pentagon was calling it a "throwback to the 1970s." The two statements are not so much inconsistent as revealing of the specific American interests that still need to be met.

I confess, too, to lingering doubts about how the president will act in the home stretch. The idea of negotiating with Moscow cuts across the whole Reagan foreign-policy thrust of extending the domain of freedom -- the "Reagan Doctrine." Negotiations, to be serious, must necessarily point toward terms that Soviets as well as Americans find to their advantage. If they bite, it puts Reagan in the position of helping to strengthen communist rule. There is a reason to do it -- to serve the American interest -- but the result is bound to leave Ronald Reagan squirming in ways hard to predict.