MANAGUA, NICARAGUA, JAN. 23 -- Sandinista government leaders are treating next month's vote in the U.S. Congress on aid to the Nicaraguan rebels as one of their most crucial political moments since their revolution won power in 1979.

While stressing that Congress' decision will do much to set the course of Nicaragua's future, the Sandinistas also have clarified the limits of democratic interplay they are willing to allow in an effort to sway the vote.

In a speech late yesterday before a crowd of 15,000 supporters, Bayardo Arce, a top ideologue of the Sandinista National Liberation Front, challenged the country's small, beleaguered civilian opposition, saying Sandinistas would remain on a "national political alert," ready to respond quickly to political developments, until Feb. 3 and 4, when the vote is expected.

Hours after rock-hurling Sandinista crowds broke up an opposition event yesterday, Arce belittled opposition leaders as "marionettes of Reagan's imperialism." Although the government lifted an almost six-year-old state of emergency this week in compliance with a Central American peace plan, Arce offered the opposition a chance not to reorganize but to repent:

"This is a time for those who have chosen the wrong path to go straight. If they do, we will receive them with a pardon," said Arce, one of the nine top comandantes who run the ruling Sandinista party. He categorically ruled out the changes to 17 clauses of the 1987 constitution that the opposition has demanded to separate the Sandinista party more clearly from the state.

He emphasized the Sandinistas' view that the party is "the people in power," and therefore "the FSLN will never turn over power because we won't usurp the right of the people."

His statement contrasted sharply with assurances by President Daniel Ortega during a recent visit to Washington that if the Sandinistas were defeated in an election, they would move into an opposition role.

The contrast between statements made by Ortega abroad and by Arce to Sandinista militants at home demonstrates how the six-month peace process has stretched the Sandinistas' flexibility to the limits. Their key decisions are made by a sometimes stormy but remarkably resilient search for consensus in secret meetings among the nine top Sandinistas. But the peace process frequently has required Ortega to respond alone, making concessions on the spur of the moment under international pressure as he did last week at a regional summit in Costa Rica. The other four Central American presidents pressed Ortega to give the civilian opposition greater freedom.

It has fallen to more radical Sandinistas, such as Arce and Interior Minister Tomas Borge, to digest the changes Ortega agreed to and make them palatable to rank-and-file party faithful who are sometimes even more insular and militant than their leadership.

Security police from Borge's Interior Ministry arrested 11 opposition leaders last week after they met in Guatemala with top leaders of the U.S.-backed rebels known as contras. Arce is in charge of the party apparatus that in the past controlled the so-called turbas, or mobs, such as the one that disrupted the opposition ceremony yesterday.

Borge has said he views the new freedoms under the peace accords as an avenue for the United States to create a clandestine urban network in Nicaragua using the civilian opposition to assist the contras. But opposition leaders have repeatedly insisted that they do not accept the contras' recourse to armed warfare.

Under interrogation by security police, several opposition leaders who attended the Guatemala meeting admitted that at least one unidentified American who appeared to be in an official capacity joined the gathering in its final hours, opposition sources and Sandinista officials said.

During the meeting, the contras attempted to persuade the opposition leaders to sign a joint statement, but they declined. But that information was enough to feed the besieged Sandinistas' darkest suspicions of conspiracy.

Some skeptical Sandinista leaders also believe that many efforts they have made since the Central American peace plan was signed Aug. 7 had little effect in Washington. For example, Latin American diplomats who were members of an international commission that assessed the progress of the peace accord before the meeting in Costa Rica last week argued that Ortega's largest concession at the summit went virtually unnoticed.

Ortega allowed the international oversight commission, which included many Latin American diplomats not strongly sympathetic to any of the Central American countries, to be dissolved and replaced by the foreign ministers of the five Central American nations.

"There is no longer any impartial source that will, as our report did, establish a balance in the picture of who has complied" with the peace pact terms, said one exasperated diplomat. Honduras, he noted, took no major measures to oust the contras from its territory. But Ortega was pressured into making new concessions in Costa Rica without succeeding in focusing any criticism on Honduras for its lapse.

In the final days before the congressional vote, Sandinista leaders have gone from emphasizing the new political leniency they will offer if the contra military aid is turned down to issuing warnings about the sharp clampdown that will come if it is approved.

Rafael Solis, a leading Sandinista official in the National Assembly, wrote in an impassioned letter to the pro-Sandinista newspaper El Nuevo Diario, "Let's be clear. If the war funds are passed, the political spaces opened up by the {peace} accords will necessarily have to be closed, to guarantee the first right of every Nicaraguan: to defend Nicaragua from foreign aggression."