The deadline for declaring official results in last month's elections in the Dominican Republic passed without action yesterday, raising concern here and in Santo Domingo that an attempt is under way to alter the seemingly overwhelming victory of the opposition party.

"This is a clear case of rigging an election. It has the aroma of the Trujillo dictatorship 20 years ago," said former deputy assistant secretary of state Ben Stephansky, who has led an unofficial Washington watch over the Dominican situation since the military intervened briefly in the vote count after the May 16 election.

Rep. Donald M. Fraser (D-Minn.) called national Security adviser Zbigniew Brzezinski to urge "that the White House take appropriate action." President Carter made a public call last month for the Dominican voters' will to be honored after the initial military intervention.

Three-term President Joaquin Balaguer, who lost the election by a wide margin in the unofficial count, had made a public promise while in Washington last year that the election results would be binding.

Several of the 35 generals in the 11,000-man Dominican army have made clear their reluctance to see the left-leaning Dominican Revolutionary Party take power, and troops briefly interrupted the vote count when its candidate, Antonio Guzman, took a commanding lead.

A 30-day period in which the electoral board is mandated by law to declare a winner ended yesterday, but the interim chief of the board said a large number of challenges by Balaguer's party required use of an emergency extension permitted by the law.

While Balaguer congratulated Guzman in what was taken as a statement of concession, his party has demanded "complementary elections" in areas where the May 16 results are challenged.

Opposition leader Jose Francisco Pena Gomez, said by telephone from Santo Domingo yesterday that the Revolutionary Party will organize "peaceful resistance" if the electoral board - which he said is dominated by Balaguer followers - calls for a new vote.

Balaguer's party successfully challenged the chief of the election board, Manuel Castillo, who had declared there were no voting irregularities.

It has since become clear that errors in the mechanized voter registration rolls prevented some Dominicans from voting but Pena Gomez pointed out that "this affected both sides."

According to U.S. officials and other Dominican sources, the lengthy delays by the electoral board could indicate an effort by Balaguer's backers to reverse the results in some key congressional districts so his party could retain a majority in the upper house, even though it lost the presidency.