WARSAW -- When Poland held local elections four years ago, Zbigniew Bujak, then the fugitive leader of the Solidarity underground, led a campaign urging Poles to boycott the vote.

This year, as the same elections come up again, the charismatic workers' leader has switched tactics. Bujak is now the chief of Solidarity's above-ground Warsaw executive, and since an amnesty 18 months ago has faced little threat of arrest. The government of Gen. Wojciech Jaruzelski, meanwhile, has launched a new program of liberalization and pledged to open this year's elections to a wide variety of candidates.

Four years ago Bujak managed to persuade more than a quarter of Poles to disdain the official candidates for local government. This year, he says, he would like to get some of his own people elected.

"We have a new situation," he said. "People aren't prepared for another boycott. They want different solutions. If we could win one or two of these town councils we could use them to address people's everyday problems."

At present, the chances that communist authorities will allow Bujak's supporters to compete in the balloting for district and provincial councils in June appear relatively small. Yet the union leader's success in participating in the official political process during the coming months may be crucial to the future of the Eastern Bloc's largest opposition movement.

After years of stubbornly resisting Jaruzelski's "normalization" of Poland following the 1981 declaration of martial law, Solidarity and the dissident political movements clustered around it have reached a crossroads.

Faced with Jaruzelski's modest reform initiatives and the atmosphere of change created by Soviet leader Mikhail Gorbachev, many of the union's key leaders have begun to advocate a shift of tactics from underground-based intransigence to using the opportunities offered by liberalization to press for more change.

The shift of attitude could mean a breakthrough in Poland's chronic polarization between communist authorities and a society wavering between hostility and indifference. But first, it poses the crucial test of whether the modest changes under way here and in much of the rest of the Eastern Bloc are creating a real or only illusionary expansion of freedom.

As Bujak's Warsaw Solidarity organization pursues its strategy of participation, that key question of legitimacy has spurred one of the most passionate and potentially divisive debates ever to occur within Solidarity. Much of the movement, including national chairman Lech Walesa, has opposed part or all of Warsaw's policy.

"The situation is very uncertain," Bujak said. "The proposals on how to act are very different. Now we simply have to go forward and see which position is proved right."

In the end, the opposition argument may be decided by Jaruzelski, who must either accept or bar the union's way into the political establishment. Since initiating his policy last October, the general has promised a major expansion of democracy. He oversaw the adoption in December of a broad platform of political liberalization by the communist party's Central Committee.

Nevertheless, the shape of the promised reforms has tended to shift erratically from week to week, apparently moved by battles within the communist establishment as well as by Jaruzelski's penchant for rethinking.

A plan to allow a partial legalization of opposition groups as "debating clubs," for example, appeared in the first draft of the political platform, was deleted before the Central Committee's approval, then apparently was revived in recent speeches by party leaders.

More confusion was created last month when government spokesman Jerzy Urban, responding to a call for a meeting between Jaruzelski and Walesa, made a statement that mixed conciliatory and antagonistic rhetoric. "Urban's answer essentially was 'yes, no and maybe,' " said a liberal party official. "That may be the best summary of the situation now."

Behind the official waffling, say Jaruzelski's associates, is a strategy of seeking to make room for moderate opponents within the political system while avoiding the compromise of party authority that marked the 1980-81 Solidarity era.

"In the end, everything will be 50-50," said one veteran party official, "because real steps must be taken and yet every step must be taken with security. We will look to build a partnership with a lot of the active people in Solidarity, but we must also make them understand that the events of 1981 cannot be repeated."

Solidarity leaders reply that no governmental move toward the opposition, however well intentioned, will be effective if it stops short of recognizing Solidarity's right to exist. "The government can talk to lots of people but in the end they must talk to Walesa," said Bronislaw Geremek, a top union strategist.

"With Walesa, by recognizing Solidarity, they have a chance to make people believe the country is really changing," Geremek said. "If they don't work with Solidarity, no one will believe in the political or the economic reform and we will see the beginning of the end for Jaruzelski."

While most opposition leaders support that stand, the growing debate within the union concerns its own plans. Its ranks of conservatives, led by the Gdansk chapter, say the union should focus on maintaining shadow factory committees and underground publishing operations and ignore legal, officially sanctioned forums like workers' "self-management" committees in factories, the election campaign and debating clubs.

Behind this stance is the conviction of a number of key opposition strategists, such as historian Adam Michnik, that Jaruzelski's government is already doomed and that, in the era of Gorbachev, it is likely to be replaced by one more ready to come to terms with Solidarity.

In contrast, Bujak's Warsaw chapter has concluded that whatever Jaruzelski's prospects, the hold-fast strategy is causing the opposition to wither. Solidarity cannot ignore legitimate new oportunities for activity, even if they fall short of its goal of legal recognition as an independent union, the Warsaw chapter says.

"Solidarity is slowly becoming a historical fact, a holy memory and a ritual gesture," said a Warsaw chapter statement in December that provoked a continuing uproar within the union. "The movement needs a philosophy of diverse activity."

To some extent, the policy struggles of Solidarity and Jaruzelski's party are converging on the upcoming elections, which have caused the union to debate whether to participate and the government to consider whether it should allow the opposition in.

The communist party's political platform initially promised that the election rules would be rewritten so that official candidates received no built-in favoritism and independent candidates could be nominated from the floor at preelection meetings open to the public.

When the first formal proposal for electoral regulations emerged from the Jaruzelski-led Council of State last month, however, the key nomination reform had been dropped, apparently eliminating any chance that Solidarity could place its candidates on ballots.

Following weeks of debate, party sources now say the election procedures will once again be changed to allow some kind of open nomination process. If so, Bujak's Warsaw organization says it will organize "local initiative groups" to place candidates on ballots.