SAN SALVADOR, NOV. 23 -- President Jose Napoleon Duarte released new evidence today linking rightist leader Roberto D'Aubuisson to the murder of Roman Catholic archbishop Oscar Arnulfo Romero, the first breakthrough in seven years in El Salvador's most important human rights case.

In a press conference, Duarte read testimony given here Friday and Saturday by Amado Antonio Garay, 37, who said he drove the getaway car for a still unidentified, bearded assassin who shot Romero as he was saying mass in a hospital chapel on March 24, 1980.

Garay's testimony says D'Aubuisson ordered the assassination of Romero and that cashiered captain Alvaro Saravia, now living in the United States, was involved in carrying it out.

D'Aubuisson, a former National Guard major who heads the rightist Republican Nationalist Alliance political party, later told reporters that the accusation was an attempt by Duarte to shift attention away from the country's problems, and he challenged the president to a public debate. Previously he had charged that the Duarte government was paying witnesses to testify falsely against him in order to ruin him politically.

Duarte said that steps were being taken to obtain the extradition of Saravia and that Garay left El Salvador Saturday for an undisclosed neighboring country, where he was in protective custody. D'Aubuisson is accorded immunity from prosecution as a deputy in the National Assembly. For him to be tried, he would have to be stripped of that immunity by a two-thirds vote of the assembly.

Duarte's dramatic announcement of a breakthrough in the case came shortly before another political rival, leftist leader Guillermo Ungo, arrived, ending seven years of exile to resume political work inside the country. Ungo, the president of the Democratic Revolutionary Front (FDR), an unarmed political coalition allied with the Farabundo Marti National Liberation Front (FMLN) guerrilla organization, was met at the airport by about 1,000 supporters, some waving red flags.

Another leftist politician, Ruben Zamora, vice president of the FDR, returned to San Salvador Saturday.

Ungo and Zamoro addressed a crowd of several thousand cheering, flag-waving supporters in front of the Metropolitan Cathedral and said their return marked a "new age" in politics here.

Both men fled El Salvador in 1980 under death threats from right-wing extremists.

The testimony in the Romero case and the return of Zamora and Ungo are seen as helping to bolster Duarte's argument that he is building democracy in El Salvador. At the same time, however, new threats by suspected death squads have raised political tensions here and served to remind Salvadorans that open leftist political organizing can still be dangerous.

Mario Reni Roldan, a partner of Ungo and Zamora in a newly formed coalition called the Democratic Convergence, told a news conference today that his life had been threatened by a death squad this morning.

In his separate news conference, Duarte denied that the disclosure of the testimony in the Romero case was timed for political purposes to distract attention from the return of Ungo and Zamora. He said the government had been trying for months to persuade Garay to come here and give his testimony to government investigators and that the timing of his return at the weekend was a coincidence.

According to the testimony, Garay was present when Saravia went to D'Aubuisson's house three days after the killing of the archbishop. Garay said he overheard a conversation in which the two discussed the murder.

"The captain said to the major, 'We carried out our plan; we killed Archbishop Romero,' " Garay said in the testimony, copies of which were handed to reporters at Duarte's press conference. Referring to D'Aubuisson and Saravia, Garay added: "The major said, 'You should not have done it yet,' and the captain said, 'We did it because you ordered us to.' "

The murder of Romero, an outspoken critic of the U.S.-backed government and military, galvanized the left into action and called worldwide attention to the civil war in El Salvador.

The day Duarte took office in 1984 he prayed at the tomb of Romero in the cathedral and promised to investigate the murder and five other prominent human rights abuse cases. The Romero killing was exempted from a broad amnesty that went into effect Nov. 5, absolving the perpetrators of "political crimes" committed by the military and death squads as well as by Marxist guerrillas.

"I can say now that I have not only fulfilled my promise to Romero, but to the Salvadoran people to get to the bottom of this heinous crime," Duarte said at the press conference at Ilopango airport, where he was awaiting the arrival of Uruguayan President Julio Sanguinetti.

Duarte said Saravia is living in the United States and that El Salvador will ask for his extradition. He said it is up to a judge to order the arrest of D'Aubuisson based on the new evidence.

"The investigation commission and the government have done their jobs; now it is up to the judges," Duarte said.

Asked whether D'Aubuisson was the "intellectual author" of Romero's murder, Duarte said, "That is a conclusion you could draw." He said Garay had passed a lie-detector test and had signed the sworn testimony.

D'Aubuisson was cashiered as a major in charge of the National Guard's intelligence unit after he was implicated in a failed coup in 1980. Saravia was expelled from the military at the same time. In February 1984 congressional testimony, U.S. ex-ambassador Robert White said the embassy had received information in November 1980 indicating "beyond any reasonable doubt" that D'Aubuisson had ordered the killing of Romero.

In 1984, D'Aubuisson ran for president against Duarte, who narrowly won with 52 percent of the vote. D'Aubuisson was elected as an assemblyman of his ARENA party.

Garay testified that he was ordered to drive a four-door red Volkswagen on March 24, 1980, and not told where he was going.

In the back of the car was a tall young man with a black beard and a rifle, Garay said. When they reached the Divine Providence chapel, where Romero was saying mass, the gunman ordered Garay to stop and pretend he was fixing the car, Garay said.

"I heard a loud shot and saw the bearded man with a rifle in his hands pointed in the direction of the church," Garay testified. "He told me to drive away slowly and quietly, and we returned to Saravia's house."

Garay said the assassin never left the car, but fired through the open door of the chapel from the back seat, killing Romero with a single shot. He said that the man spoke with a Salvadoran accent and that he never saw him again.

Romero was killed a day after urging soldiers to disobey their commanders if the orders conflicted with the law of God. "In the name of God, in the name of the suffering people whose laments arise to heaven each day more tumultuous, I beg you, I ask you, I order you in the name of God, stop the repression," Romero said.

While the driver's testimony raised prospects that the assassination might finally be solved, leftist political leaders said the new death-squad threat was aimed at scuttling their recently formed leftist political coalition and preventing Ungo and Zamora from resuming their work inside the country.

Roldan, the secretary general of the Social Democratic Party, said today that three or four armed men visited his daughter's house this morning to look for him. At the time he was elsewhere accompanying Zamora, he said.

"It worries me because death squads work like that," Roldan said of the visit by the armed men. "I'm frightened to death."

Roldan, a medical doctor, said the visit was the third death threat he has received in the last 10 days. The first, he said, occurred when two men entered his office, drew revolvers and warned him to "stop what you're doing" or become "the next after Herberth." He said the reference was to Herberth Anaya, the head of a leftist human rights commission who was murdered by unidentified gunmen here Oct. 26.

Roldan said the threats were "a response to the Democratic Convergence," the newly formed political coalition of his Social Democratic Party with the Popular Social Christian Movement headed by Zamora and the National Revolutionary Movement of Ungo. Special correspondent Douglas Farah contributed to this article.