When President Reagan goes over the heads of an angry Congress to the American public in his TV appeal for aid to the Nicaraguan contras, he ought to take Alfonso Robelo with him -- and let Alfonso do most of the talking.

Robelo was one of three contra leaders delivered to the White House press room as part of the administration's high- pitched, below-the-belt campaign to frighten (or shame) congressional critics into voting another $70 million of military aid and $30 million of "humanitarian aid" to the contras over the next 18 months. It was show-business-as-usual: real, live "freedom fighters" put on display to make the administration's case.

Surprisingly, Robelo made a different and far better case. It deserves a more careful hearing than it got, if only by way of illuminating what's missing in the administration's case: candor, realism and a decent respect for contrary opinion.

Robelo was at pains not to stiffen opposition by impugning the patriotism of congressional critics. "I can never say that a congressman of the United States would be in favor of the Soviets," he declared, showing a sensitivity and civility not shared by his patron, Ronald Reagan, who finds it "hard not to" equate opposition to his policy "with support for Sandinistas."

Scoring the right point against congressional performance, Robelo simply argued that a "yo-yo policy" of on-again-off-again military aid makes a fair test of "contra" capabilities impossible. Robelo's sense of what would be a fair test makes his case stunningly honest -- by comparison with the administration's -- if not necessarily more promising.

Mincing no words, Robelo lays out a strategy with a clear objective: by sustained military pressure, he would not aim to promote some half-baked negotiation with the Sandinistas over power sharing, the acceptance of democratic forms, the rejection of Marxist-Leninism and all the rest of the administration's pipe dreams. He will admit what the administration cannot bring itself to admit: his aim is to throw the Sandinistas out. How, given the heavy odds? Not by overwhelming military might, but by a strategy that might have some realistic hope of success. If Congress will vote the $100 million for 18 months, Robelo says, "we can define the Nicaraguan situation because we can produce internal conditions so (that) there will be defections, so there will be a clear signal to the Nicaraguan people that we can win."

Robelo may be wrong about the magnetic pull of a "contra" effort made credible by demonstrable military success. But that's what is so refreshingly forthright about his approach. If he is proven wrong, he would be prepared to say so, and pack it in: "If we don't define the situation in 18 months, this revolution will be already eight years old and I don't think we have the right to continue any further with the bloodshed in Nicaragua."

You will note no wild, scare talk about "strategic disaster" (Ronald Reagan), no "vision of two, three, many Nicaraguas -- a hemisphere of burning churches, suppressed newspapers and crushed opposition" (Secretary Shultz). Robelo would simply accept the grim existence of a Sandinista regime. That would leave it to the United States and Nicaragua's neighbors to contain the external Nicaraguan threat by diplomacy, sanctions or whatever means, but not by the cynical exploitation of Nicaraguan "freedom fighters" to advance their own, wider security interests.

That's an interesting proposition, coming from a handpicked witness in defense of the administration's case. My sense is that it is unlikely to work and that the kindest course would be to cut the contras' losses now. But it is a better course than the third alternative, which is, alas, the likeliest to prevail: a compromise that would give the administration too little of what it is demanding, too late.

That's almost a certain prescription for prolonging the agony to no purpose; for more bloodshed and atrocities and innocent victims on both sides; for putting no theory of the case to a fair test.