SOLIDARITY is about to drop the other shoe.

Its first, in the first round of its first national congress early in September, fell on sensitive Polish and Soviet toes, being close to an open demand for political power. The Polish Communist Party--no longer the traditional lackey, remember--rose stiffly to the challenge and, to Soviet growls, warned that Moscow might cut the aid that keeps the Polish economy (barely) afloat. Clearly, what the party and Moscow both want is to ensure that at the second round of its congress opening today, Solidarity finds another place to drop that second shoe.

Perhaps it will. There is a streak and a constituency in Solidarity, a democratic organization not easily controlled from the top, that keeps forcing the pace. This produced in early September, among other things, a demand for full worker self-management. That the Moscow-installed command economy of the past 30 years must be overhauled is almost everywhere acknowledged in Poland. But to go to full worker control, at once and by referendum, struck a good number of Solidarity people as too far too fast.

A proposal for a modified system has now come from the Solidarity executive and is to be put before the membership. The proposal has been greeted by the authorities as "a step toward realism"--yesterday the Polish parliament endorsed a measure in that spirit. At the same time, a Solidarity figure's catchy but risky appeal for a union-party-church coalition government seems to have been overlapped by an official proposal for a union-party- church "front."

Many people in the West hesitate to offer Solidarity cautionary words that the Soviets or Polish hard-liners must put to their own uses. But this is too difficult a time for friends of Solidarity and Poland to swallow their best judgment. The direction of Polish developments seems sound: it is good, for instance, that the debate is now centering on the critical issue--unavoidably, a political issue--of how to run and reform the economy. But the rhythm of Polish developments, entailing constant surges of action, tests of will and retreats at the brink, brings heavy risks and costs.

There is a desperate need for a quieter, more deliberate concentration on the economy on the part of Solidarity and the authorities alike. For either side to force ever-more-ultimate tests of political strength is to assume an awful responsibility. That course could produce the utter inner collapse that would amount to Poles doing the Soviets' dirty work for them.