A tentative plan for Nicaragua's main opposition group to participate in national elections has fallen apart. The plan - which called for opposition leader Arturo Cruz Porras to run as a presidential candidate in elections postponed until January - unraveled Tuesday when Bayardo Arce Castano, a member of Nica-ragua's Sandinista directorate, refused Mr. Cruz's request to have two or three days to explain the arrangement to the ''Coordinadora,'' the alliance of opposition groups he represents.
The two political leaders had tentatively agreed to the electoral arrangement Monday night in delicate negotiations conducted at a meeting of the Socialist International in Rio de Janeiro.
Socialist International leaders such as former Venezuela President Carlos Andres Perez, who was instrumental in the Rio negotiations, and former West German Chancellor Willy Brandt are urging the Sandin- istas not to let the talks break down because of a question of two or three days' timing.
However, Mr. Arce told reporters Tuesday after the agreement broke down that elections would be held on Nov. 4 without the Coordinadora's participation.
Mr. Cruz says he still accepts the ''agreement'' and hopes that the Sandinistas will reconsider their position.
The official Sandinista position is that the last deadline for registration of presidential candidates was midnight Oct. 1 (changed from Sept. 30). Having changed the registration date several times already, the Sandinistas are refusing to change it anymore.
The Sandinistas are believed to be vulnerable to some outside pressures from the Socialist International and from liberal Democrats in the United States Congress.
After the US cut off aid to Nicaragua (1980-81), much of its foreign assistance - beyond that provided from the Soviet Union and East bloc - has come from countries whose leaders are affiliated with the Socialist International, including Mexico, Venezuela, and France.
Liberal US Democrats have some leverage over the Nicaraguans, at least for a brief period, because Congress will again be considering the question of aid to the rebel contras who are fighting the Sandinista government.
The Reagan administration wants to continue aid to the contras, but it has been unsuccessful so far, largely because much of the Democratic majority in the US House of Representatives opposes the aid.
According to an aide of a Democratic representative who is playing a key role in blocking the aid:
''If the Sandinistas refuse to come to an agreement of such major political importance, merely because of a technical question of two or three days' timing, very few people will continue to believe in their good faith and they will seriously alienate any support they have in Congress and probably cut themselves off from the Socialist International, thus ending their one major source of support in the Western world.''
This aide says that several congressmen are expressing their concern to the Sandinistas.
Another figure who is trying to reach an agreement between the Nicaraguan factions is Colombian President Belisario Betancur, who has been prominent in negotiations between the Sandinistas and the opposition.
Mr. Betancur, who is not a member of the Socialist International, is one of the leaders of the Contadora nations that have negotiated a draft regional peace pact that the Nicaraguan leadership has agreed to sign.