It's a snap, if you're of a certain disposition, to fault the idea of covert proxy intervention in Nicaragua. Enough good arguments, not to speak of liberal clich,es, are available. The trick is to deal with the fact that an intervention is actually under way. Are we ready to pull the plug on real people who started something expecting that we would go all the way?

The point was brought home to me in recent talks with leaders of the two principal exile groups, Adolfo Calero of the Nicaraguan Democratic Force and Alfonso Robelo of the Democratic Revolutionary Alliance. Both opposed the dictator Somoza and then resisted Sandinista totalitarianism--Robelo as a member of the junta--until being forced into exile. They turned to insurgency when the Sandinistas refused to hold the elections they promised or otherwise let democratic forces join the political play.

Calero's group is the one that Post reporter Chris Dickey recently accompanied into Nicaragua. It has several thousand men in the field and is supported by the CIA. Calero is at pains to contest the "collective smear" that his comrades are old Somoza supporters and "criminals." Most of the emerging evidence appears to support him. (Calero is aware that his outfit, unlike some of the CIA's favorites of years past, must pass the demanding muster of American public opinion.) His group never thought the Sandinistas would engage in a political dialogue, he says, but it is open to a democratic solution.

Robelo's group, with some hundreds of guerrillas now operating on home soil, aparently does not receive or desire CIA backing, but fears it would suffer if the other's American support were cut off: the Sandinistas could then refocus their military effort and the Nicaraguan people would feel abandoned. He does not expect to achieve a military victory, he says, but to win "a seat at the table."

It is not so easy to look these courageous and patriotic partisans of democracy in the eye--"freedom fighters," President Reagan's term, fits--and explain why you think it is wrong for the administration to be trying to weaken the grasp of a Marxist regime sustained by adversaries of the United States.

It is a lot more difficult to regard covert CIA activity and the whole general idea of intervention as unacceptable, especially in the tender Central American context, when men like Calero and Robelo condone CIA participation and invite certain forms of American intervention--aid in the background. Robelo states that things are different now: the Sandinista regime exists on a foreign base--Cuba and behind Cuba the Soviet Union--and the United States would only be evening the odds.

Nor would I have the heart to say that it's all right to let down the guerrillas because Congress signed on, in 1981, only to interdict arms flowing from Nicaragua to El Salvador, not for the more ambitious purpose of knocking off the Sandinistas.

I think nonetheless that we are in the wrong business supporting an invasion of Nicaragua. The reasons are essentially practical. The intervention inhibits us from asking and getting other Latins to protest in full voice and in full diplomatic vigor against Nicaragua's sponsorship of the insurgency in El Salvador. It keeps Mexico, Venezuela and other democratic Latin countries--all with strong lefts--from making more progress with their desperate effort to edge the region toward accommodation. It lets the Sandinistas play the powerful anti-American card.

The wiser course would be to stress an aspect of American policy which Ronald Reagan upholds formally but which his CIA operation and his latest rhetoric undercut. "We do not seek (the Sandinistas') overthrow," he said on April 27. "Elections--in El Salvador and also in Nicaragua--must be open to all, fair and safe." On this platform--creation of a political process that would moot the question of intervention--Reagan would be joined by the Latins who count. It is the most acceptable and feasible way for us to do our duty by Nicaragua.