Prudent politicians are averse to answering hypothetical questions. They have problems enough with questions posed by immediate events. And in diplomacy, studied ambiguity about the future is often prudent. But there are moments, and this moment of decision on aid for Nicaragua's contras is one, when Lord Curzon's axiom is apt: Know your own mind and make sure the other guy knows it too.

The hypothetical question that the president and everyone voting on the issue should answer is: What if Costa Rica is next?

If Congress kills aid to the contras, it will kill the last impediment to the consolidation of Sandinista Stalinism. Suppose the Sandinistas mean what they say about waging a "revolution without borders." Suppose their military and destabilizing capabilities are aimed next at Costa Rica, which is democratic and unarmed. What then? The president and opponents of his aid proposal should be specific. Perhaps the president should propose for Costa Rica the kind of guarantee Britain gave Poland in 1939, and every congressman and senator should say whether he or she approves of such a pledge; and if not, why not.

It is one thing to vote against aid for the contras on grounds that they cannot succeed or the Sandinistas are not as dangerous as they are cracked up to be. It is something very different to vote against the contras while also flinching from answering the question, "What if the Sandinistas are that dangerous?"

Such a pledge by the president would strengthen what the small size of his aid request subverts -- a sense of seriousness. Anyone opposing both aid for the contras and the pledge to Costa Rica would stand reasonably suspected of believing that the idea of a negotiated taming of the Sandinistas is just a convenient excuse for flinching from hard facts.

As to the president, there is a stark disproportion between what he says and the reasons he gives for saying it. He rightly casts the issue in the language of his State of the Union reference to the "Soviet drive for domination." He rightly compares the contras to the Hungarian freedom fighters of 30 years ago. He rightly calls the overthrow of the Sandinistas, or at least an effective prophylactic meas essential to hemispheric stability.

He has painted the picture in the strongest primary colors, but his request is a pale pastel: just $100 million, only $70 million of it for military aid. A request five times larger might be more successful because it would be more realistically related to the scale of the task and the president's assessment of the stakes.

This is not the first instance of a debilitating disproportion between the administration's intense characterization of an issue and a tentative commitment to action. The president says the Strategic Defense Initiative is a moral imperative and a prudential necessity. However, he is not even committed to deploying it. Indeed, his administration has made a fetish of the ABM Treaty, which must be changed if SDI is to exist.

Many critics of aid to the contras say that the president has failed to produce negotiations with the Sandinistas. There they go again, blaming America first. Can they not credit the Sandinistas' sincerity as revolutionaries?

Contra leaders, pushed into contortions by Congress, say they will have no moral right to continue if they cannot do the job in 18 months. But what is the job? The contras suggest it is to get the Sandinistas to the negotiating table. But setting such a deadline tells the Sandinistas how long they must stay away in order to win. Anyway, communist regimes have been stains on the planet for 68 years and never has one been talked down from totalitarianism to pluralism.

A sufficient reason for funding the contras is independent of a belief that they can win a military victory or compel the Sandinistas to accept diplomatic cauterization of their festering infection in Central America. The reason is bleak but serious: we should support any struggle that burdens the Soviet imperial system.

We fund the Afghan resistance although there is no realistic hope that Afghanistan will be anything other than integrated into the Soviet bloc. We do it to maintain a debilitating fever in the Soviet system. The contras can contribute to a better world by delaying the day, when Sandinista power is consolidated and the dynamic of Stalinism turns, as it eventually must, outward.

If people (democrats, ideally, but political hygiene is not the point) are willing to die shooting at appendages of the Soviet empire, it is prudent for us to buy the bullets.