Three months ago, the Polish Communist Party took the historic step of conceding workers the right to form independent trade unions. Today it faces another acute problem: how to control the growing demands for democracy among its own members.

Attacked and criticized from all sides, the Polish party is as dispirited and divided now as at any time during its 35 years in power. Just how far it can and should reform itself is likely to be the major theme of an important two-day plenary session of its Central Committee which begins Monday.

If the party moves too far or too fast, its leader Stanislaw Kania risks the same fate that befell Czechoslovakia's Alexander Dubcek in 1968. One of the reasons why the Kremlin has not intervened in Poland so far is that it still trusts the Polish Communist Party to retain its grip on society.

On the other hand, if the party fails to reform, it risks further antagonizing the Polish people. The communist leaders face the prospect of hanging onto the trappings and instruments of power, but of being obeyed by nobody.

Given these difficult choices, observers predict that Kania will again try to steer a middle course. His problem is that, in trying to please everybody, he may please no one.

The plenum will take place against the background of last week's dramatic events in Warsaw that illustrated the emerging divisions within the new Solidarity union federation between hardliners and moderates. It took all-out persuasion by Solidarity's leadership to get the Warsaw steel workers to call off a strike over demands for the reform of the security services.

Kania faces a similar raft within the Communist Party. The party apparatus, together with the 140-member policy-making Central Committee, remains relatively conservative and resistant to change. But demands for major reform have been welling up from Poland's 3 million party members, many of whom also belong to Solidarity.

The kind of reforms involved is illustrated by a motion passed by basic party organizations in Lodz. It said: "The party must not be a monolith of false unity nor a pyramid erected for the glory of the chosen ones."

It said the leadership "is not working fast enough and is always behind events. Either it does not understand the dangers of the present situation or it lacks political competence."

The motion called for fully democratic procedures in all party bodies, an emergency party congress by January at the latest, and the election of an entirely new leadership untainted by the mistakes of the past. The party, it said, should put itself at the head of the revolutionary changes taking place.

Such calls are likely to meet determined resistance within the Central Committee, many of whose members would be swept away in genuinely democratic elections.

A sign of who is on top will come from leadership changes that emerge from the meeting. They are not expected to be startling, but observers are watching to see if the reformist faction in the Politburo is strengthened by the inclusion of men such as Tadeusz Fiszbach, the party chief in Gdansk, or Mieczyslaw Rakowski, the editor of the weekly magazine Polityka.

The former interior minister, Mieczyslaw Moczar, has also been mentioned as a strong candidate for re-election to the Politburo after nine years in the political wilderness. But some observers believe he would prefer to return in triumph as a result of a groundswell of opinion at a party congress rather than from a closed Central Committee meeting.

Moczar, a nationalistically inclined politician, has recently been regaining some of the influence he enjoyed in the late 1960s, when he led an "anti-Zionist" campaign.

Two matters of recent history are likely to come up at the plenum. The responsibility of the former party chief, Edward Gierek, and the former premier, Piotr Jaroszewicz, for the present crisis will be discussed. Gierek suffered a mild heart attack in September shortly before his dismissal and has not been seen in public since.

The party is also expected to decide its attitude toward the bloody suppression of workers' riots along the Baltic Coast 10 years ago.At least 50 persons were killed when police opened fire, but full details of the casualties and the government's handling of the affair never have been made public.

Solidarity has demanded publication of an official report. A commission was appointed in 1971, but its findings never have been disclosed.

Publication of a report, and the naming of the officials who gave the order to use force, would enable the authorities to participate in huge commemorative ceremonies to be held in the ports of Gdansk and Szczecin on Dec. 16, the 10th anniversary.