The Carter administration is weighing plans to send U.S. Army advisers and up to $7 million in military sales and credits to El Salvador this year in an effort to prop up its faltering new government and block a leftist takeover, according to official sources.

The proposed security assistance program, put together over the past several weeks to complement an emergency $50 million in economic aid also planned, has not yet been accepted by the Salvadoran government.

But it already has caused deep division within the State Deparment where many specialists on Latin America say it marks a return to U.S. counterinsurgency techniques long used against the Latin American left but largely discarded as policy in the early 1970s.

The program calls for at least three 12-man Army mobile training teams to instruct each of El Salvador's three Army battalions in logistics, communications and intelligence techniques. A six-person team was sent to El Salvador for five days last November, along with $200,000 worth of tear gas, gas masks and bulletproof vests, to give instruction in riot control.

The plan's critics argue that growing instability in El Salvador, ruled by a military-civilian junta that replaced a rightist military regime last Oct. 15, would lead to ever higher levels of U.S. military support and create a "quagmire effect" similar to the gradual U.S. involvement in Vietnam in the 1960s.

Proponents of the plan say stronger internal security against left- and right-wing violence in El Salvador is the only way of assuring that the current U.S.-backed moderate government can carry out vital social, economic and political reforms that all agree are the only remaining option for avoiding civil war.

Sources said the was tentatively approved by the National Security Council, with strong backing from presidential security adviser Zbigniew Brzezinski and the Pentagon, last month. Since then, they said, objections have arisen within the State Department and Secretary of State Cyrus Vance is expected to question the plan at an NSC meeting scheduled for Friday.

In addition to upgrading the Salvadoran armed forces arsenal the proposal calls for training the already primarily U.S.-equipped and trained military to be less repressive and avoid human rights abuses. Because of the sensitive nature of the program, both here and in El Salvador, the administration has sought to involve other countries, including West Germany, Spain and Venezuela, in the security assistance effort.

The strong emphasis on internal security would represent an abrupt change in U.S. Latin American policy that appears to be a direct response to the global cold war atmosphere in general, and in particular to increased political upheaval in Central America.

Although the administration apparently has decided it can "live with" what so far has been a relatively moderate and friendly leftist government in Nicaragua following the Sandinista overthrow of Anastasio Somoza last summer, it believes the Salvadoran left is more extreme and would shift the political balance in the region.

"We would be faced with swallowing a hard pill," one administration official said. "I don't know if we'd get it down or not."

In addition to the internal State Department battle, State disputes some of the facts of the current situation in El Salvador with the Department of Defense. In public testimony before Congress last week, Deputy Assistant Secretary of Defense Franklin D. Kramer said there is "evidence of Cuban efforts to orchestrate communist movements in Central America. There are also indications that the Cubans currently appear to be involved in the situation in El Salvador."

The State Department officials indicate that no persuasive evidence of Cuban involvement exists beyond training of Salvadoran guerrilas on Cuban soil. They agree that Defense has strongly pressured for implementation of the program and for stepped-up U.S. military involvment in central America.

Sources said a similar contingent of advisers has been approved for Honduras, but the teams have not yet been sent.

"The Pentagon's objective ultimately is to get the Marines in there," said one official who strongly opposes the program, "and show that they can win a guerrilla war."

Implementation of the program has been put off pending its acceptance by the Salvadoran government. The five-man coalition junta is composed of two Army colonels and three civilians -- one independent and two from the Christian Democratic Party.

The civilians, sources said, have resisted the plan because of its potential political liabilities. The question, a U.S. official said, is "how do they accept security assistance without being branded imperialist stooges?"

The Salvadoran civilians suggested the involvement of other Western countries in a multilateral assistance effort. Venezuela, whose Christian Democratic rulers sympathize with their fellow party in El Salvador, has been involved in discussing with the United States.

Last week, William Bowdler, assistant secretary of state for Latin America, traveled to Spain and West Germany to discuss their participation. aThe Spanish, whose embassy in Guatemala was burned with 39 people killed last month and whose embassy in San Salvador currently is being held by leftist militants, reportedly expressed interest in the plan. West Germany reportedly did not.

The Salvadoran military "wants us" to provide the assistance, sources said.

El Salvador has not received large-scale U.S. military aid since its former all-military government angrily refused it in early 1977 in protest against U.S. human rights policy. In fiscal 1976, the U.S. government authorized weapons sales and credits totalling $1.1 million to El Salvador.

While threats from the left have grown, dissentaion between civilian and military officials and the increasing possibility of a right-wing coup pose a perhaps more immediate problem in El Salvador.

Until last October, El Salvador had spent more than 40 years under an uninterrupted series of military presidents elected in what were often considered rigged contests. The military was supported in power by a small economic oligarchy that controls the vast majority of the wealth.

But by October, increasing charges of official human rights abuse, a broad-based expansion of the left's political base, mounting terrorism and the impetus of revolution in nearby Nicaragua drove the president, Gen. Carlos Romero, and many high-level military offices from power.

The young, ostensibly progressive military officers who had organized the coup invited left-center politicians to join it, and the new government pledged immediate reforms. By January, however, that government, too, collapsed as its civilians resigned en masse, charging that remaining rightists in the military had once again taken over and had blocked the promised changes.

Both the young military officers, who agreed to try again to gain the upper hand, and the United States, appealed to the Christian Democrats to form a coalition government with the armed forces. That is the government the United States now seeks to support.

The new government has been denounced by all other center and center-left opposition groups and even by elements within the Christian Democratic Party for associating with the newly discredited military. The left has vowed to overthrow it. Even the Catholic Church, while professing its support for nonviolent change, has declared the junta unrepresentative and has begun to move more toward the left.

Meanwhile, the economic oligarchy, sensing impending doom, has moved further to the right, and joined with factions in the military to fight reforms. There is growing evidence that rightist paramilitary groups allied with Guatemala's increasingly isolated military government are actively aiding similar terrorist organizations that have recently appeared in El Salvador.

Nicaragua, whose leftist government fears its relationship with the United States may become untenable should the Carter administration intervene in El Salvador, is growing nervous.

Although the new junta has tried to move rapidly toward reform, this week announcing the nationalization of banks and land reform, repression by the military it ostensibly controls has increased as has violence from the left. The Salvadoran human rights commission announced Tuesday that at least 194 people have been killed in political violence over the past two weeks.

"This government has no friends," a State Department official who supports the security assistance program admitted last week. But Bowdler reportedly told a closed session of the Senate Foreign Relations Committee two weeks ago that it is the only remaining hope for avoiding civil war and a Marxist government in El Salvador.

The last time large numbers of military advisers were used in Central America was in Guatemala during the late 1960s, when the United States tried to prop up a similar moderate civilian government that had replaced a military leadership.

That government eventually failed and was replaced by the predecesor to the current military government amid charges that the United States has participated along with the Guatemalan armed forces in repression of the left. p

Critics within the department say no limited involvement is now possible, and that the premises on which it is based -- that the junta is willing and capable of reform, and of appeasing both left and right -- are faulty.

These critics argue that the only way for El Salvador to survive is for it to move quickly toward the moderate left, with a government taking control of the military, excluding it from power and providing a role for center-left and popular leftist organizations.

"If we are prepared to back a government that shaky," one official against the program said, "we have to look at what our long-term options are down the road. I believe that 75 percent of this is political within the United States -- a fear by the administration that it will be accused of 'losing' something else, like it 'lost' Iran, Nicaragua and the Panama Canal."