President Alvaro Magana of El Salvador warned congressional critics of President Reagan's Central America policies yesterday that "a weak, vacillating commitment" to his government in its fight against leftist rebels threatens the future of democracy in the entire hemisphere.

Magana also assured Reagan in a White House meeting that he is working to curb human rights violations in El Salvador and that no U.S. troops or additional advisers are now needed there, a senior administration official said afterward.

But Magana was questioned closely by members of Congress earlier yesterday about the status of the prosecution of Salvadoran national guardsmen accused of murdering four American churchwomen in 1980, according to congressmen attending the meeting. They said Magana was told that human rights violations in El Salvador threaten congressional approval of more U.S. aid to his government.

Magana, 57, told reporters after his meeting with Reagan that continued U.S. aid is essential to achieve a "lasting peace through democracy."

"El Salvador fights not only for the survival of its own democratic system," Magana said; "we also defend western democracy. For this reason I want to appeal to the honorable members of the Congress to support the efforts of President Reagan to aid El Salvador.

"This assistance strengthens the cause of democracy in the Central American region," he added. "A weak, vacillating commitment endangers peace and hemispheric security. For this reason the people of the United States must fully understand that we face a common threat."

Reagan stressed in a statement that Magana "reaffirmed his government's commitment to human rights," and said El Salvador is preparing to hold new democratic elections.

"President Magana is a courageous and talented leader," Reagan said. "He's making admirable progress in the difficult task of moving El Salvador toward democracy while at the same time coordinating a defense against Marxist-led guerrillas who would turn his country into a Cuban-style dictatorship . . . . The government of El Salvador and the people of that brave country deserve and have our support."

While Magana met with Reagan, State Department spokesman Alan Romberg said requests from Salvadoran rebels for negotiations with Magana's government should be rejected because it "seem to be a demand for power-sharing. The government of El Slavador and U.S. policy have repeatedly rejected power-sharing as an acceptable means of political reconciliation in that country."

Magana reportedly told members of Congress in a breakfast and meetings on Capitol Hill that the trial of the national guardsmen for the murder of the nuns would begin within six weeks.

"Yes, they made questions . . . about the general problem of human rights," Magana said as he left the Capitol. "What I tried to convince them is that we are facing very difficult problems and we would like to go faster but it is difficult in a period of violence."

Meanwhile, Nicaragua's new ambassador to the United States, Antonio J. Toledo, said yesterday that Secretary of State George P. Shultz should meet with Nicaraguan Foreign Minister Miguel D'Escoto because of "interesting and constructive remarks" Reagan made when accepting the ambassador's credentials Thursday.

The White House would not elaborate on the conversation. A State Department spokesman refused comment but suggested it was not useful for Toledo to make the meeting request public.

In another development, Vice President Bush, speaking to the Inter-American Defense College, an Organization of American States school that trains military officers for Latin American countries, said every democratic nation in Latin America is threatened by communism.

"To what extent the insurgents are actually controlled by the Soviets and the Cubans we cannot know for certain," Bush said, "but there can be no doubt that they all act in concert and that their goal is the 'liberation' of the entire continent. No country in Latin America is immune."

Later, in an apparent reference to concern about human rights violations in democratic countries in the region, Bush said: "We do not demand that democracy be perfected in order to qualify for our support. Democratic government is perhaps one of mankind's noblest achievements, but it is also one of the most difficult."