Washington — The debate over the United States role in the mounting turmoil of Central America is coming to a head.
There is strong feeling here that within weeks, perhaps even days, there could be startling new developments.
On the one hand, the Reagan administration is on the verge of taking new action aimed at heading off escalating tensions in Central America. Responding to Mexican offers to serve as a ''communicator'' in damping down tensions in the region, President Reagan says he ''will welcome'' this Mexican help.
On the other hand, the State Department indicates the US is ''just about ready'' to impose some sort of sanctions on Cuba for its alleged role as the source of some of Central America's troubles.
Moreover, after a week in which Caribbean and Central American themes dominated much of Washington's attention, administration spokesmen are going on the offensive against critics of Reagan policy in the region.
Washington refuses to rule out the possibility of US military intervention in Central America or the Caribbean. But Mexican President Jose Lopez Portillo's warning that US troop intervention would be a ''gigantic historical blunder'' is being taken seriously.
The feeling that events are moving fast in Central America is heightened by a number of developments including the reported visit of Cuban President Fidel Castro to Nicaragua over the weekend. Moreover, Salvadoran opposition leader Guillermo Ungo, visiting the US, appeared on Meet the Press and called again for unconditional negotiations to end the war.
The Castro trip comes on the heels of President Lopez Portillo's visit to Managua, the Nicaraguan capital, where he made his proposal to serve as a mediator between the US and Nicaragua as well as between Washington and Havana.
President Reagan, talking with a small group of newsmen Feb. 25, said that, in addition to welcoming the Mexican proposals, he had received a letter from Mr. Lopez Portillo on the initiatives. Mr. Reagan commented, ''If there is a way in which he (Mr. Lopez Portillo) could be a help then we will welcome it.'' He added that the Mexican letter is being studied.
Other US sources say that the two Presidents were in telephone contact on the evening of Feb. 23 only hours before Mr. Reagan spoke to the Organization of American States on his Caribbean Basin proposals. It is understood they spoke at length about both the Mexican mediation and the US Caribbean initiative.
It remains to be seen how the Lopez Portillo and Reagan proposals will mesh; there are significant differences in US and Mexican views on the turmoil in Central America. The US continues to hold Cuba ultimately responsible, while Mexico puts a heavy emphasis on ''the struggles of poor and oppressed peoples to live better,'' as Mr. Lopez Portillo phrased it a week ago.
Still there is a personal warmth between Mr. Reagan and Mr. Lopez Portillo. This should help the two find some common meeting grounds on how to deal with the explosive issues of Central America. At the moment the most pressing is that of El Salvador, where an escalating civil war between leftist guerrillas and a centrist government has engulfed the small country.
US officials insist that Soviet arms are reaching the guerrillas through Cuban and Nicaraguan channels - charges that both countries actively deny.
So far the US refuses to document the charges. And President Reagan, answering a question from this reporter, said he could not give proof on the charges because this could compromise ''our sources.'' But he added: 'Believe me , we are sure of our facts.''
Official Washington reticence to discuss US options in Central America and the Caribbean was nudged aside briefly before the weekend when Thomas O. Enders, assistant secretary of state for Inter-American affairs, told a small group of newsmen in the White House that a number of measures are being prepared to deal with Cuba.
He mentioned the startup of Radio Marti, a new propaganda tool to be beamed at Cuba. And he said that other measures - restrictions on travel and shipping - are ''just about ready.''
It is understood that, for starters, the US may close down the daily Havana-Miami air service and also reactivate rules preventing ships that dock at Cuban ports from calling at US ports. These two measures are more symbolic than threatening to Cuba, but they would cause considerable inconvenience to Cuba and Cubans.