Rightist leader Roberto D'Aubuisson, twice accused of plotting to overthrow the U.S.-backed junta here, today called for the ouster of Christian Democratic civilians in the coalition government and the establishment of a full-fledged military regime.

D'Aubuisson, a former Army major in the Salvadoran intelligence service, said he believes that such a move would win the support of the Reagan administration. He said he based that assumption on extensive contacts with "members of Reagan's group" dating back to before the U.S. presidential election.

Those contacts, D'Aubuisson said, include unspecified meetings with Roger Fontaine, currently in charge of Latin America for thhe National Security Council, and Lt. Gen. Daniel Graham, a former director of the Defense Intelligence Agency, and a member of the Reagan State Department transition team with no current position in the administration.

D'Aubuisson said he thought the Reagan administration would accept or even "favor" the departure of the Christian Democrats from the junta.

Neithher Fontaine nor Graham could be reached for comment on D'Aubuisson's claims. A White House spokesman, to whom an inquiry to Fontaine was referred, said there would be "no comment for the moment" until the administration had had a chance "to study" D'Aubuisson's claims.

The Reagan administration, which yesterday announced a large-scale expansion of its military aid program here, has repeatedly stated its full support for El Salvador's military-civilian coalition government.

Acting Ambassador Frederic Chapin, according to sources in Washington, has been instructed to "assuage the military" and support its fight against leftist guerrillas, but also to emphasize U.S. backing for the Salvadoran government as currently constituted with Christian Democrat Joes Napoleon Duarte as president.

D'Aubuisson, 37, served as second in command of El Salvador's military intelligence organization under the previous government of president Gen. Carlos Humberto Romero, who was overthrown by "progressive" military officers on Oct. 15, 1979. Although he has been accused twice of plotting against the new government, which joined the military with the centrist Christian Democrats, and was briefly arrested here last May, D'Aubuisson is known to command the support of a large sector of the Salvadoran middle and upper classes, as well as many within the military.

Last July, he was deported from the United States after he entered illegally on a visa that had been revoked by the State Department because of his alleged ties to rightist terrorism here.

Since then, he has divided his time between visits to Guatemala and apparen unmolested residence in El Salvador. Today, he spoke to journalists at a table outside one of the decaying mansions in the heart of one of San Salvador's wealthiest neighborhoods.

"The Reagan administration is with the armed forces," D'Aubuisson said. "If the armed forces stay with the line of Oct. 15, [the day of the reformist coup that first won U.S. support for the government], I don't see why Reagan would be concerned."

Both the Carter and Reagan administrations have depended on the participation of the Christian Democrats in the government to ease acknowledged repressive tactics of the military and justify U.S. involvement here. But the message the Reagan administration is sending officially and the message D'Aubuisson and other Salvadoran conservatives say they have gotten appear to be two different things.

In December, when the Salvadoran government was shaken by a series of assassinations, including the murder of three American nuns and a lay worker, the Christian Democrats blamed the extreme right both inside and outside the government for trying to destablize the regime.

At the same time, then-U.S. ambassador Robert White accused members of Reagan's transition team of conveying the message to rightist leaders here that a takeover by them would be acceptable. The Reagan administration denied sending any such word.

The Christian Democrats here have accused D'Aubuisson of leading death squads responsible for the assassination of several ranking members of their party during the last year.

As he spoke today, D'Aubuisson carried a gun and a dagger, and was surrounded by bodyguards he said he needs for protection from both the leftist guerrillas and Christian Democrats. He said that friends of his in the National Police and the National Guard here have assured him that there is no warrant out for his arrest, despite his history and repeated statements against the government.

D'Aubuisson accused the Christian Democratic Party as a whole of being a communist organization.

"The Christian Democratic Party in any part of the world," he said, "is the rightist sector of the Communist Party."

Although he made it clear he wants and expects the Christian Democrats out of the current government as quickly as possible. D'Aubuisson shied away from a call for a coup.

"The armed forces could simply ask them to resign," said D'Aubuisson. "They could say, "thank you very much and now goodbye.'"

D'Aubuisson set no deadline for such an action but noted with a smile that "March is, I think, a very interesting month."

On March 11 an amnesty offered to the guerrillas by the government will expire and there is considerable speculation here that a crackdown by the government will follow. D'Aubuisson would not reply to speculation that a coup could also take place at that time.

Much of the strength of D'Aubuisson's appeal to the upper economic class here lies in his claims that the Christian Democrats are more responsible for the economic ruin of the country than the guerrillas.

D'Aubuisson's essential call is for a return to "free enterprise," and an end to the extensive economic reforms that the United States has encouraged and the current government has carried out in an effort to snatch political support away from leftist guerillas.