A senior leader of Poland's suspended Solidarity trade union movement today called for "a difficult and uncompromising" fight to counter what he said was a danger of a legal ban on the movement by the Communist authorities.

Zbigniew Bujak, the leader of Solidarity's Warsaw branch who escaped detention after the imposition of martial law on Dec. 13, also appealed to trade unionists abroad for support. He said that various signs, including the disbanding of the Polish Journalists' Association and the dismissal of democratically elected university rectors, raised the possibility of Solidarity itself being delegalized.

The three-paragraph message, copies of which were relayed to Western correspondents, appeared to mark a new turn in Poland's political crisis.

It reflected the conviction of many Solidarity members that, nearly four months after the military takeover, the union movement's existence is now in jeopardy.

Bujak, who signed the message in his capacity as a member of Solidarity's decision-making presidium, has until now favored a cautious approach to the martial-law authorities. He has argued in underground publications for the need to avoid a "final showdown" on the ground that it could lead to catastrophe.

The toughening in his position appears in part to be the result of recent events, including the forced resignation last Wednesday of the rector of Warsaw University, Prof. Henryk Samsonowicz.

Along with other trade unions and professional organizations, Solidarity was only suspended--not banned--under martial-law regulations. Its future is now the subject of a nationwide "debate" from which, however, its own supporters have very largely been excluded. Discussion has centered on whether the union should be abolished once and for all or allowed to operate under stringent political restrictions.

Bujak's message read: "The disbanding of the Polish Journalists' Association, the dismissal of democratically elected rectors, recent press articles and information reaching us all point to the fact that we face a danger of Solidarity being delegalized.

"A difficult and uncompromising fight for our union is the moral and statutory duty of all Solidarity members.

"To unionists in other countries who have always shown us friendship and help: when the time of this fight comes, use all means at your disposal to grant us support."

The debate within the government-controlled media on what to do with Solidarity has been mirrored by a similar debate in the underground press over the best form of resistance to martial law. One of the main instigators of this debate is Jacek Kuron, Poland's number one dissident and one of Solidarity's leading theoreticians, who somehow managed to smuggle his contribution out of Warsaw's Bialoleka Prison, where he is interned.

Kuron's opinions appear in the latest edition of Solidarity's underground Warsaw weekly, Tygodnik Mazowsze..

The underground paper quoted Kuron as seeing a danger of widescale disturbances. This in turn, he warned, could lead to a collapse in the government's authority and a Soviet invasion.

The only way to avoid such a catastrophe, in Kuron's view, is to create a tightly organized resistance movement with an efficient information network. Such a movement should prepare for a showdown with the authorities by "a simultaneous offensive" against all the centers of power and information.

The purpose of this well-coordinated show of strength, Kuron was quoted as saying, would be to force moderates within the party to reach a compromise with Solidarity. He did not go into details about the form the "offensive" should take.

The same issue of Tygodnik Mazowsze carried articles on the same theme by Bujak and one of his deputies, Wiktor Kulerski, who is also in hiding in Warsaw. Their criticism of Kuron's proposals reflects a more general difference in outlook between those Solidarity activists being detained and those at large.

In his article, which was probably written at the end of March, Bujak said he did not believe that "a showdown" would solve anything. Indeed there was a danger that it would provoke still greater repression and foreign intervention.

Bujak said that a centralized resistance movement, as proposed by Kuron, would be easier for the government to break. Instead he called for "a decentralized movement" in which different social groups would employ different methods of protest.