Once again Poland sees just how tentative political stability can be

Political stability in Poland seems suddenly to be in question again -- not only for the ruling generals in Warsaw, but also for the men in the Kremlin.

And more than a few diplomats here say that the official response, both in Moscow and in Warsaw, will be to advocate an even tougher line toward backers of the suspended trade union movement.

As one harbinger of this, they cite the reimposition by Polish martial law authorities of an overnight curfew following street clashes there May 3.

The curfew, imposed along with martial law last December, had been lifted only one day earlier.

Still, it is harder to guess the ultimate policy response in Warsaw and Moscow to the unrest than to identify the two likely choices the antiregime protest will present.

* The first, diplomats suggest, would be to get yet tougher against those Poles whom martial law has not cowed.

* The second would be to seek some sort of political entente with them, or with at least the more moderate of them.

One thing is clear: Poland has not achieved the ''normalization'' that martial law was supposed to be cementing.

Until the recent Polish street clashes -- which, according to Polish reports, spilled into May 4 in the northwestern port city of Szczecin after being quelled May 3 in Warsaw -- something of a surface calm had seemed to mark Polish politics.

True, the battered Polish economy remained pretty well battered, the one relatively bright spot being an upturn in coal production. The Soviets and other East-bloc states are diverting resources from their own strained economies to the Polish martial law authorities.

But whatever the drain on Soviet economic resources, Polish martial law had brought political dividends, in Soviet eyes. ''Antisocialism,'' as the Kremlin lexicon identified the more muscular reformist trends within the Polish union movement, had been curbed.

Even the Soviets -- as private remarks from senior officials made clear -- did not suppose that mere imposition of military rule would definitively crush a reformist push that enjoyed support, or at least sympathy, among millions of ordinary Poles.

But martial law, as one Soviet put it a few months back, had brought ''order.'' Lech Walesa -- who, after all, is in detention -- no longer exercised effective veto power over Poland's ''ruling'' communists.

Or, in the words of another Soviet, a senior official: ''The current Polish leadership feels, in a certain sense, in control. . . . This is something that could not be said about the preceding one.''

The recent show of antiregime sentiment in Poland -- a May Day demonstration tolerated by authorities, and the May 3-4 protests that were broken up -- seems likely to call into question such assessments.

The feeling among foreign diplomats in Moscow is that much in the ultimate reaction of both Moscow and Warsaw to the Polish unrest will depend on developments in the immediate future. Were the recent demonstrations a portent of others to come?

But the diplomats assume that Soviet and Polish officials favoring political entente as a bid for genuine stability in Poland may have second thoughts now.

Such officials do exist both in Moscow and Warsaw -- or didm before the Polish unrest.

But at least some such officials also feared that rolling back too quickly on Poland's military crackdown would invite a return to the ''antisocialism'' -- that is, reformism -- that martial law was meant to curb.

Thus, senior Soviet officials privately supportive of a bid for some kind of political understanding between the current Polish leadership and supporters of Solidarity were apt to stress that the process must be pursued cautiously, and within set limits.

In Warsaw, one Polish official is understood to have told visiting Western politicians earlier this year that the release of union leader Walesa was, for similar reasons, impossible.

Internationally, both the Soviets and the Polish authorities would seem to enjoy considerable leeway in plotting policy -- particularly at a time when much of the world is preoccupied with a conflict in the faraway Falkland Islands.