A majority of Americans are so opposed to U.S. military involvement in El Salvador that they would support young men who defied a government order to fight there, according to a new Washington Post-ABC News poll.

Six in 10 Americans say the United States has already gone too far simply by sending 55 military advisers to assist the Salvadoran civilian-military junta in its war against the rebels in that country, and more than 7 in 10 oppose President Reagan's plans to send more military equipment and weapons there.

Strikingly, this opposition to involvement in El Salvador exists even though large majorities also agree with the administration that a successful insurgency there would lead to the toppling of other Latin American governments and endanger the national security of the United States.

The poll shows that citizens tend to believe Reagan and Secretary of State Alexander M. Haig Jr. when they charge that the rebels are being armed by Cuba and Nicaragua. Those two countries are seen as interfering in El Salvador far more than the United States is, and Cuba is widely seen as intervening improperly elsewhere in Latin America as well.

Those beliefs, however, fail to budge most Americans. Only two people in 10 say the United States should help the Salvadoran junta, and seven in 10 say the United States should stay out of El Salvador entirely.

Two out of three people interviewed told The Post and ABC News that they feel the fighting in El Salvador is much like the war in Vietnam. And concern about the intentions of national leaders appears as widespread as it ever was during the Indochina war years.

Only 42 percent of those interviewed say they believe the Reagan administration when it says it has no current intention of sending soldiers to fight in El Salvador; the same number say they do not think the administration is telling the truth; the rest have no opinion.

Furthermore, by a ratio of more than 2 to 1, those interviewed say they expect the United States to send troops to El Salvador eventually, if the junta cannot defeat the rebels. More than 4 in 5 say they would oppose such an action.

Question: Just suppose the United States were drafting soldiers and sending them to fight in El Salvador: Would you support or oppose young men who refused to go?

Answer: 51 percent say they would support such resisters; 42 percent say they would oppose them, and 7 percent express no opinion.

Predictably, those who believe the administration on El Salvador tend to be those who support the president generally, while those with doubts tend to be those critical of him. Those who believe government statements that the United States has no plans to send troops to El Salvador are strong Reagan supporters, with 72 percent approving his handling of the presidency. By almost 3 to 1, they say the United States is not heading for a Vietnam-type involvement in El Salvador.

Those who doubt the government's word hold almost exactly the opposite views: 68 percent of them disapprove Reagan's handling of the presidency, and by more than 2 to 1, they see El Salvador becoming another Vietnam for the United States. A strong majority of them feel that the United States is guilty of interfering improperly in the internal affairs of Latin American nations.

The new poll, conducted Thursday evening through Sunday, also shows Americans to be intensely opposed to covert actions aimed at overthrowing Latin American governments. Three out of every four say the U.S. should not "secretly try to overthrow the government of Cuba," although a plurality--43 percent of those interviewed--agrees with the statement that "most of the revolutions and civil wars in Latin American countries are started by the Cuban government."

Two in every three say the United States should never "secretly get involved in overthrowing a Latin American government." Only one-quarter of those interviewed think the United States is trying secretly to overthrow any Latin American government.

As in most foreign affairs issues, a great many Americans are poorly informed on the situation in El Salvador, the poll shows. Twenty-one percent of the 1,218 people interviewed said they had not read or heard anything about the fighting there; among the majority who had heard of it, only two-thirds were aware that the United States is backing the Salvadoran government against the rebels