The Reagan administration's switch to all-out support for Britain in the Falkland Islands crisis has left two of Washington's most important supporters in Latin America feeling isolated and betrayed, and it could produce a dramatic change in relations between the United States and its neighbors to the south, diplomats here report.

The two nations, the democracies of Venezuela and Costa Rica, which have provided Washington with vital support for controversial U.S. policies in El Salvador, have joined other Latin American nations including the leftist governments of Cuba and Nicaragua not only in offering support to Argentina but also in questioning the U.S. role in the Organization of American States.

These side effects of the Falklands decision have undermined Washington's push to isolate Cuba and Nicaragua, and, coupled with the defeat of the Christian Democratic Party in El Salvador in U.S.-sponsored elections in March, have lessened Venezuela's strategic support for Washington's Central American policies.

Given the enormous leverage the United States exercises over Latin America and the underlying, long-term nature of hemispheric relations, diplomats caution that the effect of the Falklands crisis on North and Latin American ties should not be exaggerated. Those tacit and implicit ties are weightier and older than the formal terms of any single treaty or quarrel, and the Latin Americans' rush to Argentina's side should by no means be considered wholehearted, they add.

But for many Latin American governments the debate goes beyond the fine points of diplomacy to the deeply emotional issue of anticolonialism and the region's historic fear and resentment of U.S. intervention in its internal affairs.

"The United States chose to side with its stepmother, and not with its little brother," said Julian Montes de Oca, the spokesman for the Venezuelan Embassy here.

By choosing a European power over a Latin American one, said the spokesman for the Costa Rican Embassy, Mario Hidalgo, the United States has "completely destroyed the principles of the OAS."

The narrow legal focus of the U.S.-Latin American debate is whether the 1947 Inter-American Treaty of Reciprocal Assistance, known as the Rio Treaty, is applicable to the present crisis. The treaty calls for hemispheric military assistance to any nation threatened by a foreign power.

Only three nations abstained with the United States on the issue in the recent emergency session of the Organization of American States: Trinidad and Tobago, Chile and Colombia.

In an open letter to Latin American heads of state made public Tuesday in San Jose, Costa Rican President Rodrigo Carazo called for the transfer of the OAS headquarters out of Washington. Carazo was succeeded yesterday by Luis Alberto Monge, a social democrat who generally has voiced even more emphatic Latin Americanist stands.

The leader most directly affected by the U.S. switch is probably Venezuelan President Luis Herrera Campins. The Christian Democratic leader accepted a lot of domestic controversy and debate within his party when he became a vocal backer of El Salvador's Christian Democratic-military government, also supported by Washington. Now the Christian Democrats are out of power in El Salvador, a right-wing coalition has taken over, and the United States has abandoned Argentina's generals, who were expected to provide crucial military support for the beleaguered Salvadoran Army.

The Venezuelans are showing new caution and a wait-and-see attitude toward the new government in El Salvador, in contrast to its previous warm embrace of the government there.

"As long as the Salvadoran government shows respect for our agreements, for human rights and democracy, we will reciprocate," said Montes de Oca at the Venezuelan Embassy.

Diplomatic sources confirmed Venezuela's standing agreement to provide the Argentine armed forces with fuel "and other things that may be necessary" when the Argentines request it. This parallels the public position taken by Cuba and several other Latin American nations.

In an interview with the Mexican daily Unomasuno, published May 6, Herrera Campins reaffirmed Venezuela's "total backing" of the Argentine position with regard to the Rio Treaty. He also noted that the outcome of Venezuela's territorial dispute with Guyana, which involves almost two-thirds of what is now Guyanese territory, will depend largely on whatever precedent is set by the Falkland Islands negotiations.

Moreover, Montes de Oca said that both Foreign Minister Jose Alberto Zambrano and Herrera Campins had said in Venezuela, after the OAS ministers meeting, that a total revision of the Rio Treaty was called for.

Most OAS governments appear to have been uneasy at the prospect of having to choose between supporting the Buenos Aires junta on the one hand or seeming to accept the British "colonial" position on the other, and have restricted their participation in the debate to the pro-Rio Treaty vote in the Washington meeting.

Sources close to the Brazilian Foreign Ministry, on the other hand, report a sanguine attitude. The Foreign Ministry "feels it must defend the principle of nonintervention, but Brazil will be quite happy to see Argentina, its historical rival, take a beating," one of the sources said.

In the Latin American community, only Mexico's position is more detached than the Brazilians'. In a public statement last weekend, Foreign Minister Jorge Castaneda repeated Mexico's support for Argentine sovereignty over the Falklands, but he stressed that it does not consider Argentina's first use of force "legitimate."

Castaneda has held all along that the Rio Treaty should not be invoked in the crisis and that U.N. Security Council Resolution 502, which calls for Argentine withdrawal from the islands before negotiations, must be supported. Mexico has been surprisingly low-key about the U.S. decision to side with Britain.

"It's a problem of alliances," the foreign minister said. The ministry is reluctant to commit itself to a far-away, right-wing dictatorship while supporting the leftist armed oppposition in nearby El Salvador. Nor is Mexico--"so far from God and so close to the United States"--eager to buy itself additional quarrels with its northern neighbor.

If the crisis should degenerate into a prolonged military confrontation, however, and U.S. participation should increase, Mexico can be expected to join the Latin American outcry. In a number of Latin American countries, the conflict is having the same effect on domestic political constituencies that is it having in driving countries as diverse as Cuba and Venezuela together.

"In my country we are all together on this, Communists, social democrats, Christian democrats, conservatives, liberals, everybody," said a Venezuelan who voiced strong support for Argentina.