Mexico has launched itself into Central America politics with a major offensive to isolate Nicaraguan President Anastasio Somoza.

Having broken off diplomatic ties with Gen. Somoza's government Sunday, Mexico has now dispatched two high-ranking missions to five countries of the area - Venezuela, Colombia, Panama, Jamaica and the Dominican Republic - to urge them to suspend relations as well.

Despite the proximity of Mexico to Central America, this is the first time it has assumed a major political role in the region - perhaps a reflection of a new confidence brought by the country's massive new oil finds and its need to show it is independent of Washington.

Accompanying the Mexican envoys are representatives of Costa Rica, which broke with Somoza six months ago after border incidents. Today they are in Bogota, where the Andean Pact presidents of Peru, Bolivia, Ecuador, Venezuela and host Colombia are meeting this week.

While Mexico traditionally has ignored Central America, the small countries of the isthmus have looked to it as the "giant of the north"-much as Mexico has looked to the United States-and its decision to break with Somoza had a great impact in the region.

Nicaraguan Foreign Minister Julio Quintana admitted in an angry news conference Monday, "We had to recognize that Mexico enjoys international prestige and besides it has a lethal weapon, oil, which it can use politically."

Mexico currently sells no oil to Central America, which is largely supplied by Venezuela. But next year it will begin oil sales to Costa Rica. Still, no Central American nation is keen to alienate the Mexicans.

Independent analysts here believe the Mexican breakoff, followed by accusations that Somoza was committing "horrendous genocide," it also a clear signal to the other military-dominated governments of Central America.

The military rulers of Guatemala, El Salvador and Honduras, who have tremendous social problems of their own, are on Somoza's side in the escalating Nicaraguan civil war. To the south, Costa Rica, Panama and Venezuela are supporting the opposition movements and the guerrillas in Nicaragua.

Until now, however, Mexico had refused to take an active role. Last October, when the United States formed a regional mediation team designed to persuade Somoza to leave or accept a plebiscite, Mexico declined to participate when invited.

"I told [Secretary of State] Vance, you Americans created a monster," the then Mexican foreign minister Santiago Roel said at the time, "so you get rid of him. We don't want to be involved."

But the collapse of the U.S.-led mediation effort has sharpened divisions on the issue. Privately, many regional diplomats chide the United States for not following through on an alleged promise and toppling Somoza.

Now Mexico has taken the lead. "We hope our action may lead to other [breakoffs] and that the diplomatic and political isolation of Somoza will speed the downfall of this bloody regime," Mexico's new foreign minister, Jorge Castaneda, told a news conference yesterday.

The Mexican initiative is in keeping with its recent efforts to maintain a distance from Washington. This year President Jose Lopez Portillo gave a cool reception to visiting President Carter while warmly receiving the French and Cuban heads of state.

Fidel Castro's state visit last week was followed by Costa Rica's Sunday.

The Mexicans insist that their talks with Costa Rica and not with Cuba prompted them to break with Somoza. CAPTION: Picture, JOSE LOPEZ PORTILLO . . . unusual role in Central America