The Soviet Union is finally going public with the obvious: its satisfaction over the growing antinuclear movement in the United States.
Diplomats here are attributing the uncommon public restraint that marked early Soviet commentary on the US peace movement to concern that American antinuclear groups might be hurt domestically by overt Soviet news media support.
Moreover, the Soviets, like many US political observers, seem to have been surprised by the suddenness with which the American antinuclear movement has been gathering force. Only a few weeks ago, one senior official here began a conversation with the Monitor by asking just how serious the nuclear-freeze campaign in the US seemed to be.
A measure of the Soviets' initial public restraint remains, still affording a contrast between the official media's treatment of West European peace movements and their coverage of the American one.
But gradually, this is changing. An April 12 commentary by the Soviet news agency Tass said, ''The antiwar movement in the USA has assumed an unusual scope for the first time since US imperialism's aggression in Vietnam. The Reagan administration tried to present the results of the latest elections as the American people's mandate for an unbridled arms race. But that stratagem has failed.''
''The current antiwar movement in the USA,'' the commentary said, ''has a broader base than in the years of the war in Vietnam. It involves not only the youth, but also the 'average American. . . .' ''
Three days after the Tass commentary, the Communist Party newspaper Pravda termed an April 10 march in Chicago a ''protest against the Reagan administration's suicidal course toward building up nuclear-missile arsenals'' and a ''new stage in the peace forces' offensive in the USA.''
Pravda's April 18 edition, devoting a large chunk of a weekly international affairs review to the American antinuclear movement, said: ''Even in the United States itself the Reagan administration has realized, with unconcealed regret, that what has stimulated an unprecedented surge of antiwar demonstrations is not some mythical 'hand of Moscow,' '' but the administration's ''own bellicose words and actions.''
''Fierce (political) battles lie ahead,'' the Pravda commentary added, pointing to congressional discussion of a nuclear freeze. ''But it is already clear that the unreserved support for a militaristic course evident until recently in the capital, has been shattered.''
The presumed Soviet hope is that congressional support for some form of antinuclear move will deepen and nudge Mr.Reagan toward favoring early arms-control results.
Although the Soviet media have been paying more and more attention to the idea of an arms freeze, against the background of the freeze campaign in the US, Soviet officials continue privately to stress the need for revitalizing the recently dormant negotiating process for strategic arms limitation.
At least, as a few Western analysts here hold, the Soviets' recent suggestion of a superpower summit this October, which would be shortly before US congressional elections, may reflect a hope in Moscow that voter pressure could sway Mr. Reagan toward a generally softer stand on superpower arms issues.