The State Department launched another round yesterday in the escalating battle of countercharges about human rights violations in Nicaragua, issuing a report charging ruling leftist Sandinistas with "ever greater repression of their own people to maintain their hold on power."

Illustrating the central role that all sides assign to the human rights issue in the controversy over U.S. policy in Nicaragua, the report was timed to coincide with House debate opening today on President Reagan's $100 million aid request for Nicaraguan rebels.

In recent weeks, at least half a dozen groups, individuals and officials have taken public stands on the human rights issue, freely admitting an interest in influencing the aid vote, expected to be close.

Opponents have said that the rebels, known as contras or counterrevolutionaries, are brutal murderers who have alienated Nicaraguans, cannot win their support and so do not deserve U.S. help.

Aid proponents, echoing Reagan, have said some abuses were committed by Nicaraguan troops disguised as contras. They argue that the Sandinistas must be pressured by armed rebels before they will change practices that, as yesterday's report put it, have "systematically violated" a pledge to respect human rights while establishing a totalitarian state.

On Feb. 27, an outspoken backer of Reagan's proposal, Rep. Robert K. Dornan (R-Calif.), told reporters, "This debate is going to depend heavily on the relation of stories of human rights violations and their believability."

He appeared with Alvaro Baldizon, a Nicaraguan government defector who has become a controversial figure in the human rights debate.

Baldizon said his job as special investigator of human rights questions for the Sandinista interior ministry involved covering up abuses rather than exposing them.

He was first to level, albeit without proof, charges about disguised Sandinista troops, alleging that the government had killed 2,000 political opponents, often through a "special measures" bureau of assassins operated by Interior Minister Tomas Borge.

Reagan and the State Department report cited Baldizon's charges. But Americas Watch, a New York-based human rights group, subsequently reported that Baldizon has altered his stories to a point that "eliminates the last vestige of credibility."

Baldizon initially said high-ranking officials ordered 90 percent of the 2,000 murders. Then he told congressional committees that many were massacre victims in isolated villages, Americas Watch said. Baldizon also said he had learned about Nicaraguan government drug-running in 1984 but later said the year was 1983.

In a letter to Baldizon, Americas Watch said Nicaraguan human rights groups have documented about 300 killings by Sandinistas over the last six years and noted that all of Baldizon's examples were drawn from those 300 cases.

Baldizon's contention that six times that many murders occurred is "not credible," the organization reported March 4.

Baldizon was traveling yesterday and unavailable for comment. But John Blacken, acting director of the State Department's Office of Public Diplomacy, which is paying Baldizon's expenses, said there were no inconsistencies.

Baldizon said 90 percent of the 700 cases he investigated, but not all 2,000 deaths, were officially ordered, Blacken said.

Baldizon said he found evidence of drug trafficking in 1983 but only became convinced of it in 1984. When he offered additional specifics on the alleged murders to Americas Watch, the organization rejected them as unverified, Blacken said. "They wanted it both ways," he said.

The administration views human rights violations as secondary to the question of Sandinista "support for subversion and imposition of a Marxist-Leninist totalitarian state," Blacken said. "The human rights violations derive from their imposition of this kind of state."

To critics of U.S. policy in Nicaragua, however, alleged human rights violations by contras are crucial.

Rep. David E. Bonior (D-Mich.), chairman of the House Democratic Caucus committee on Nicaragua, said the issue is "complex and contradictory and hard for Congress to sort out. But it is a factor" in the aid vote, he said, "because it responds to Reagan's perception of the contras as the Founding Fathers."

A noncontroversial part of Reagan's request would provide the contras' human rights commission with $3 million, a sum Americas Watch vice president Aryeh Neier said would make them "the world's second-best funded human rights group" after Amnesty International.