The victory of the Dominican Republic's opposition party in the May presidential election has been weakeded by an Electyoral Board decision giving the current ruling party control of the Senate.

While confirming the election of Antonio Guzman Fernandez as president over incumbent Joaquin Balaguer, the Electoral Board ruled Saturday that four contested seats in the country's powerful Senate would go to Balaguer's Reformist Party. The ruling gave the Reformists a 16-11 majority in the Senate, which they could use to block Guzman's legislation or possibly bring down his government.

The Revolutionary Party said it will contest the Board's decision on the Senate seats in the courts before Guzman takes office, a party spokesman said, their government may try to reverse the Board's decision.

Control of the Senate is particularly important in the Dominican Republic because the Senate appoints all judges, including the chief justice of the Supreme Court who would assume the presidency if the president and the vice president died or were deposed.

"At best this is a design for instability for a country already facing severe economic problems, and at worst it can set the stage for anu kind of later move against the government," said Ben Stephansky, former U.S. deputy assistant secretary of state. Stephansky has led an unofficial Washington watch over the Dominican situation since the military intervened briefly in the vote count after the May 16 election.

The election became something of a test for President Carter's human rights policy when he expressed "serious concern" about the vote-counting process. Balaguer had promised Carter that the election would be a model for other Latin American countries.

A source in Washington who follows Dominican politics, said the Board's decision - Which followed the dismissal of its chairman Manuel Castillo and seven weeks of deliberation - was "manipulate by Balaguer."

The Revolutionary Party made plans yesterday to call on all Dominicans to stay home today. The plans were canceled, a party spokesman said, when it was learned that government-backed rightist gangs known as las bandas would use the occasion to conduct a campaign of "terrorism and looting" in Santo Domingo, the capital.

State Department officials said they had no indication that the Electoral Board's decision was dishonest and added that they hoped power would be transferred to Guzman peacefully as scheduled.