Moscow — The United States invasion of Grenada is shaping up as a political windfall for Moscow, which had been thrown on the political defensive in recent weeks. The crisis is bound to be particularly welcome in the Kremlin at a time when senior officials have been voicing increasing bitterness over the Reagan administration and, privately, increasing concern that they would likely be dealing with Mr. Reagan for another term.
The Caribbean crisis could also strengthen the Soviet Union's hand in its public bid to forestall deployment of new US missiles in Western Europe.
But most diplomats here feel it is premature to speak of any major Soviet gain on that front - and doubly premature to speak of ''good news'' over Mr. Reagan's reelection prospects.
In practical terms, the importance of Grenada to Soviet security interests is seen by foreign envoys here as considerably less than the US administration has suggested. The pin-dot island is also much less important to Moscow than its larger Latin American ally, Cuba.
Indeed, when a Soviet television announcer reported the violent coup in Grenada earlier this month, the map projected behind him showed not the Caribbean, but the area around the Spanish city of Granada.
The broadcasters apologized for the error.
Yet politically, the US-led invasion has lent Grenada far more than pin-dot significance in Kremlin eyes.
A major, short-term reason is that the invasion - and subsequent criticism of Mr. Reagan internationally - has at least temporarily supplanted pressure on Moscow over its downing last month of a crowded Korean jumbo jet.
The US seems this week's least favorite superpower.
For Kremlin foreign-policy specialists - whose primary areas of concern are inevitably superpower and East-West relations - the evident hope is that President Reagan will find it harder to get out of Grenada, politically and militarily, than to get in.
This is particularly true at a time when the Soviet news media have been predicting, with some signs of enjoyment, that the US is getting similarly bogged down in Lebanon.
Lebanon, much more than Grenada, is an area of Soviet security concern. It is , as officials are quick to note, a region much closer to Soviet borders. It also directly involves Moscow's most important Arab ally: Syria.
But the situation there, from the Soviet point of view, is less critical than in the immediate aftermath of last year's Israeli invasion.
Says one Arab diplomat: ''Syria is in a much stronger position to call the shots than a year ago. . . . Plus, the Americans have not confirmed Moscow's concern they'd be able to consolidate the power of a stable, pro-Western government there.
''The Soviets - and in a sense, the Syrians - are in the nice position of being able to sit back and watch the US get more and more bogged down.''
Against this background, Moscow's response to both the Lebanon and Grenada crises is so far centering on a gradually escalating verbal campaign to heighten political pressures on Washington. The campaign will inevitably include invective by the Tass news agency, and sharply worded statements at higher Soviet political levels.
What practical effect either crisis will have on long-term superpower and East-West relations is seen as depending in large measure on US and Western political developments.
It is generally felt by diplomats here that, at least as things now stand, neither crisis will by itself derail the December deployment of new US missiles in Western Europe.
The Soviets may find it easier, in seeking to encourage Western antinuclear groups, to play peacemaker against Ronald Reagan's ''warmonger.''
But at present, diplomats feel the gain will not greatly change the Euromissile equation without announcement of a further, substantive change in the Soviet arms-negotiating position as the planned deployment date nears.
A Western diplomat, reflecting a widely held view, adds: ''Much of the effect of the Grenada crisis will depend on how soon and cleanly the invasion force withdraws. . . . And on how the political aftermath is handled on Grenada, and in Washington.''