THE CHANGE at the helm in Poland was, fortunately, a Polish choice, not a Soviet one. Out went Stanislaw Kania, the career functionary elected Communist Party leader by secret ballot in July; he succumbed to party divisions and the difficulties of the situation. In comes Gen. Wojciech Jaruzelski, defense minister and, since last winter, prime minister. The very appointment of a military man--nowhere else is this done in the Soviet bloc-- is a stunning official no-confidence vote in the party and an effort to transfuse into it the popular standing still enjoyed by the armed forces. It leaves Gen. Jaruzelski with all the formal power that it is within the capacity of the traditional political structure to bestow. After him, there is, it seems, no one.

The crux of the problem is, of course, that the workers have created an alternative power structure. The government and party work the old levers, but Solidarity has disconnected or loosened them at the other end. That leaves the authorities caught between an unacceptable Soviet solution and an unacceptable American solution. They can't crack down, as the Kremlin keeps urging, without risking massive unrest and resistance. Gen. Jaruzelski seems to recognize this: he is known for his pledge not to call out the armed forces against the workers. Neither can they follow the Reagan administration's advice to engage in "negotiation and compromise" without calling their authority further into question and inviting workers to raise the ante of their challenge.

It is a fateful struggle, the more so that it is not strictly speaking a struggle between black and white. The workers have the priceless advantage of legitimacy, but they have yet to show the self-discipline that is its vital complement. The authorities cannot claim to represent the same popular will, but they are acting nonetheless out of an evident devotion to some of the highest needs of the Polish nation. That both sides have such credentials is what gives the contest between them its undeniable tragic quality.

In cruel circumstances of economic distress and political polarization, the Poles must find a way to serve the aspirations for liberty championed by Solidarity and the aspirations for order and continuity represented by the powers that be. Otherwise, the whole Polish experiment will collapse, without the Americans or anyone else's being able to sustain it and without the Soviets' firing a shot.