MANAGUA, NICARAGUA, OCT. 4 -- At a press conference held last week by Nicaragua's main opposition coalition, a shouting match erupted between journalists and politicians over what groups were members.

The leaders of the Democratic Coordinating Group were outraged that the reporters did not already know all the exact names of the 14 member organizations so they refused to repeat them. When the yelling crescendoed and the leaders finally relented, it was easy to see why the reporters were confused.

The Coordinating Group includes the Social Christian Party and a splinter group, two warring factions of the Conservative Party, one branch of the Liberal Party and six chambers of commerce.

Not included are two other Conservative Party factions, one of which supports the leftist Sandinista government, nor the other Social Christian breakaway, nor one Liberal Party -- which is off on its own -- nor a smattering of left-wing microparties that think the Sandinistas are too conservative.

The regional peace plan signed Aug. 7 in Guatemala gives new importance and international support to the opposition parties here. Based on a proposal by Costa Rican President Oscar Arias, the pact seeks to bring peace by encouraging rebels to lay down their arms and make way for the democratic opposition parties to grow under new freedoms the government must allow.

But Nicaragua's opposition is more divided and powerless than at any time since the Sandinista-led 1979 revolution, party leaders and western diplomats said.

On Monday, the opposition is facing a major test of its new stature under the peace plan, when it will begin an open-ended national dialogue with the government, one of the measures called for in the pact.

The government wants to limit the dicussion to how the peace accords should be applied in Nicaragua. The main opposition groups will bring ambitious demands, including the separation of the ruling Sandinista party from the state and the armed forces, party leaders said this weekend.

In two short-lived attempts at national dialogue, in October 1984 and September 1986, tensions within the opposition coalition ran almost as high as those in their conflict with the Sandinistas.

The parties suffered because of a longstanding Nicaraguan tradition of infighting combined with constant harassment by the Sandinista ruling party, several leaders said. They also noted that for seven years the Reagan administration, anxious to oust the Sandinistas rather than press them, backed the armed rebels over the democratic opposition.

"The political parties have been decapitated and disoriented by the Sandinistas. They cut us off from the people and used us as window dressing," said Alberto Saborio, secretary general of the largest of four Conservative party factions.

Two Conservative factions are so small they are jokingly referred to as "sofa parties," meaning that all their members can fit on one couch.

The full spectrum of 15 parties includes 11 that are legally registered. The Sandinista National Liberation Front dwarfs the others. Since the late 1970s the Sandinistas, moving behind the scenes, have persuaded dissidents from the Conservative and Independent Liberal parties to break with their leadership and join in tactical alliances -- giving these dissidents more power in the Sandinista-dominated national legislature.

Parties that remained in the opposition were infiltrated and restricted under a state of emergency from holding open demonstrations.

On Sept. 27, the Social Christian Party celebrated its 30th anniversary with meetings in two different Managua movie theaters three blocks apart. One meeting had the blessing of the international Christian Democratic organization. The other was staged by a renegade faction that broke away in 1986 over a power dispute.

Many veteran Social Christian members at the second meeting, who traveled to Managua from remote rural villages, said they were unaware their leaders had split or that another celebration was under way. One 18-year-old who eloquently expounded the sins of the main leaders identified himself to a reporter with a slip of the tongue as a member of the Sandinista Party youth organization.

Social Christian Secretary General Agustin Jarquin said his party's split was also due to "Latin-style personality flaws in a country with a red-hot political situation."

Meanwhile, one influential Social Christian leader, Azucena Ferrey, and a prominent Social Democrat, Luis Rivas Leiva, gave up on the prospects for democratic politics in Nicaragua early this year and joined the contras, as the rebels are called.

Sandinista leaders continue to regard the opposition with suspicion and not a little contempt. The vice minister of interior, Luis Carrion, who is also one of nine top Sandinista commanders, recently called the opposition "the self-appointed representative of the contras."

Carrion said at a press conference that the parties have no interest in expanding their membership and only cultivate "good relations with the gentlemen of the U.S. Embassy."

The weekend brought intense inter-party arguing about the upcoming dialogue. "Their morale is on a roller coaster," remarked a diplomat trying to follow it.

"We know that behind every Sandinista move is a trap," Coordinating Group president Carlos Huembes said.