The diplomatic mood in Latin America has tipped against Washington in recent months, weakening the Reagan administration's sales pitch to Congress for aid to the Nicaraguan rebels and isolating the United States in the region on the issue.

Seven South American countries and Mexico, grouped in the Contadora movement, took the unprecedented step last month of formally urging the U.S. government to halt aid to the guerrillas and press efforts for a regional peace treaty.

These countries are convinced that such U.S. involvement only aggravates the conflict in Central America and hurts chances for a negotiated settlement there, according to senior diplomats of Latin American and other western countries.

The Latin Americans' stepped-up role has its roots in the expansion of the Contadora movement last summer, in the emergence of new democratic governments in South America, and in a new sense of regional solidarity owing to the Latin American debt crisis, diplomats said.

In Central America, a center-left tide in recent elections has contributed to fresh interest in peace talks with Nicaragua's Sandinista government.

Costa Rica has irritated the United States by talking directly with Nicaragua about the two nations' border problems. Guatemala's new civilian government, hoping to make a foreign policy splash, plans to host a summit of Central America's five presidents in May.

The diplomatic trend appears to have undermined Washington's efforts to isolate Nicaragua. Instead, the United States finds itself isolated in the face of nearly unanimous regional opposition to helping the contras, or counterrevolutionaries, who are fighting a civil war against the Sandinista government.

"The United States' support efforts for the contras lead to concern that it could be the cause of destabilization and greater conflict in Central America. There is real ideological agreement on this among the countries in the Contadora groups," a South American ambassador said.

The Latin Americans' shift has attracted the attention of U.S. allies in Western Europe, Canada and Australia and reinforced the allies' own skepticism over U.S. policy in Central America, western diplomats said.

Nevertheless, the tilt is unlikely to have much effect beyond its impact on U.S. and foreign public opinion. None of the Contadora countries has expressed any intention to cut back economic or diplomatic ties with the United States because of disagreements over Central America.

Moreover, Washington may benefit later this year from the Latin American countries' own growing frustration with the Nicaraguan government's military buildup and curbs on political pluralism, according to the Latin and other western diplomats. These sources emphasize, however, that dissatisfaction with the Sandinistas did not translate into support for the contras.

To some extent, the Latin Americans are making a fuss over contra aid in order to underline their opposition to direct U.S. military action in the region, western diplomats and other observers said. Several Latin American governments worry that use of U.S. forces against Nicaragua would trigger large protest demonstrations and cause significant political unrest in their countries, the sources said.

Peruvian President Alan Garcia said Friday that his country would break diplomatic relations with the United States if it attacked Nicaragua.

"The Latins are saying very strongly that the time has come to tell the United States that you can't just send the Marines into Latin America anymore," a senior West European diplomat said.

As far as contra aid is concerned, U.S. officials argue that the Latin Americans' diplomatic stance is largely a public relations gimmick playing on anti-Yankee feeling at home.

"Central America is a long way away from some of these Contadora countries and is not a priority national interest," a U.S. official said.

Some U.S. officials say that representatives of several Latin American nations have told the United State privately that their governments "secretly" favor a tough line against Nicaragua, and even want Washington to back the contras. This was disputed strongly, however, in interviews with senior diplomats of four Latin American countries and three non-Latin U.S. allies.

"It is unrealistic for the United States to say that all of our diplomatic work is just for show. This attitude leads nowhere," a South American envoy said.

A western ambassador said: "The Americans claim to hear privately all the time that people wish they would go in against the Sandinistas. That is just not what we are being told by the Latin Americans."

A high point of Latin American opposition to U.S. policy in Central America came on Feb. 10, when the foreign ministers of the eight countries of the Contadora movement met jointly with Secretary of State George P. Shultz in Washington to urge the United States to halt aid to the contras.

The original Contadora group -- Mexico, Venezuela, Colombia and Panama -- has been trying since January 1983 to draw up a region-wide peace treaty for Central America. The group, named for the Panamanian island where it first met, has produced two major draft treaties calling for Cental America's countries to reduce their military arsenals, halt all support for guerrillas in neighboring countries, and promote democracy at home.

Neither pact has won universal acceptance among the five Central American nations: Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica. Critics charge that the Contadora process has yielded only meaningless paperwork and fruitless diplomatic conferences.

But the Contadora process got a boost last July when Brazil, Argentina, Uruguay and Peru formed a support group for Contadora. The Contadora movement now includes every major Latin American democracy except Bolivia, Ecuador and the Dominican Republic, and represents approximately 300 million people.

Three of the support group countries -- Brazil, Uruguay and Argentina -- have switched recently from military to civilian governments, which the U.S. administration applauded.

In general, however, the new democratic governments are pursuing policies that are more nationalistic and more critical of Washington.