THE PRESIDENT'S Nicaragua plan is in trouble. It amounts to warning the Sandinistas to negotiate to American satisfaction in 60 days or to face a full new American commitment to support the contras and their cause. And there is reason to believe that what Mr. Reagan is asking is not simply that the Sandinistas respect their neighbors but also that they accept an internal process leading to their fall from power. The Nicaraguans are unlikely to accept that. So Mr. Reagan's policy points to a harder military collision. Congress is right to resist it.

But what does Congress intend to put in the Reagan policy's place? It is right to be careful about a drift toward deeper proxy intervention and perhaps direct American intervention. But it is also important to be careful about a right-sounding but essentially deceptive drift toward "diplomacy." Congress, having wisely undertaken to limit the Reagan policy, may be moving unwisely toward something that is a policy in name only. Diplomacy has got to have some teeth in it. A policy made up wholly of enticements -- promising more trade, for instance, if negotiations advance -- and without prospective sanctions -- making no provision to restrict trade if negotiations flag -- is not a serious policy.

The first need is to keep the Sandinistas from subverting neighboring countries or endangering broad American security interests. At the same time, the Sandinistas can reasonably ask that their neighbors not facilitate intervention by the contras. To arrange such an exchange of obligations is the purpose of the Contadora group. It has been working to some effect on the key enforcement provisions, and it must work harder.

But so long as the Sandinistas and many of their friends and enemies have the idea that the true Reagan purpose is to overthrow the Sandinistas, the regional diplomacy is going to drag, and the Sandinistas are going to regard any call for internal talks as a maneuver serving that larger Reagan purpose. So it is essential to create conditions that will draw the Sandinistas into a political proc that could have some positive results. A cease-fire would be a big help, but two other steps are also necessary.

The president should be willing to dem over time that he can live with a Sandinista government that is -- let's be realistic -- moving toward openness. Congress should be willing to demonstrate that it will make the Sandinistas pay a price for not moving toward openness. Again, to imagine that the Marxists in Managua will head toward social democracy if only the wicked contras are taken off their back is fooling.

President Reagan and Congress are now engaged in a tense confrontation over the terms on which certain funds may be provided for the next five months. On both sides they are thinking too small. The deal that remains to be defined and struck entails a curtailment of presidential ambitions and an assertion of congressional responsibilities. Mr. Reagan needs to back away from military intervention just now and agree to live with the Sandinistas on fair terms. Congress should support or make possible the sanctions -- cuts of trade and investment, diplomatic boycotts and other political sanctions -- that would give the Sandinistas a strong incentive to abide by fair terms. These at least are the fundamentals of an above-board, realistic and honest policy. They promise better than anything currently on the board.