Rightist leader Roberto D'Aubuisson vowed today that El Salvador's new government will control the left-wing guerrillas in "no more than six months" while fulfilling "assurances the United States wants" to keep aid flowing.

The most prominent member of the newly dominant right wing said at a news conference that the government will include members of the Christian Democratic Party, but not President Jose Napoleon Duarte.

D'Aubuisson, a cashiered Army major and head of the Nationalist Republican Alliance (ARENA), showed uncharacteristic moderation not only regarding the Christian Democrats but the guerrillas. He said "everything will be worked out" between current political factions during the coming week so the constituent assembly can convene just after Easter, with all sides in agreement on the form of the interim government it is to create.

"We will share the destiny of our country with the PDC the Christian Democrats , but in no way will we accept Duarte as head of the government," D'Aubuisson said. He denied any plans to arrest or otherwise persecute Duarte. "We have no intention of seeking revenge on anyone," he said.

Duarte, now leader of the military-backed junta installed by a coup in 1979, indicated yesterday he would be willing to step down if his party demanded it. D'Aubuisson is unlikely to be in the leadership either because of strong opposition to his views in the United States, but both men will be major figures on the political scene no matter what their positions. D'Aubuisson was elected a member of the new assembly.

He denied having received any U.S. pressure to moderate his tone and said the new government would "continue and increase the assurances the United States wants" to keep aid flowing to El Salvador.

He added he plans to visit Washington as part of a delegation when the government is consolidated. Assistant Secretary of State Thomas O. Enders said yesterday that previous restrictions on D'Aubuisson's entry have been removed.

Talks among the five parties that won seats in Sunday's elections will open Sunday or Monday on the structure of the new government leadership, D'Aubuisson said. He was uncharacteristically evasive on what his party's demands will be.

About 6,000 people attended a Christian Democratic rally in Liberty Park near the center of San Salvador to hear speakers warn against the danger of "an effort to steal the election from us again." Duarte is widely acknowledged as the winner of 1972 elections canceled by the military.

One senior party official, Antonio Guevara Lacayo, said. "We have no problems sharing the government with all the parties of the country because we have always intended to govern for all the people."

The crowd showed little enthusiasm until Duarte began to speak. But he told them the rightists "are puppets managed by the oligarchs living out of the country" and if those forces tried to set back reforms begun by the Christian Democrats, "that would be to attack the history of the nation."

If his party is excluded from the government, "I am sure the people of El Salvador will be with us here," said the president as he looked out over the park and the crowd shouted back, "Duarte, Yes! Another, no!"

D'Aubuisson said the new government would pass an amnesty law with incentives that he did not specify for guerrillas to surrender and rejoin Salvadoran public life. "But if they insist on drawing blood from our nation, the last resort will be to throw them out of the country," he said.

D'Aubuisson included an economic recovery program in his list of measures to stop the violence, noting that El Salvador now has at least 500,000 unemployed people, or more than 10 percent of the population. Controlling the guerrillas, he said, will take "no more than six months. It is hard to say there will be absolutely no violence, but there will be complete control . . . . What they can do then will be insignificant."

D'Aubuisson's relatively conciliatory tone toward both the Christian Democrats and the guerrillas contrasted with his hard line during the long election campaign that climaxed Sunday with a 1.3 million turnout--a figure of 1.5 million issued yesterday apparently having been discounted today by the election commission.

D'Aubuisson on the stump repeatedly held up watermelons labeled Christian Democratic Party and then sliced them in half with his machete, saying that like the watermelon, the party was green--its identifying color--on the outside and red on the inside.

Then his followers would sing his campaign song: "El Salvador will be the tomb where the Reds will meet their doom." He said he would "clean up the guerrillas within three months." His party took 383,600 votes to win 19 seats in the 60-member National Assembly, second only to the Christian Democrats' 526,000 and 24 seats.

D'Aubuisson had a classic explanation for the turnaround: "To campaign is one thing, to govern is another."

At the hot and crowded party headquarters, D'Aubuisson fidgeted in his chair most during a discussion of the rightist coalition's plans for the economy and for existing land and banking reforms. "We will try to make all the reforms more productive," he said, "but 35 decrees on land reform have caused a lot of trouble among the people. The details will have to be corrected."

He appeared to back down from previous declarations that nationalized banks should be returned to private enterprise, saying that "more people should be included in the benefits" of the banks, but that the government "will not allow a small group of stockholders to control the system." Separate banks would be set up for laborers, industry, farmers and coffee growers, he said.