If the president doesn't get the money he wants for the Nicaraguan contras, White House Communications Director Pat Buchanan wrote the other day, the Sandinistas will first crush the contras and soon "they will be in San Diego." And that's not half of what Buchanan had to say recently in The Post (op-ed, March 4) and on "CBS Evening News." If the contras are defeated, the United States will have two options: either we "stand aside and watch the Warsaw Pact roll up Central America or we send in the Marines."

And if we don't send in the Marines, and Central America falls? The Soviets' Bear bombers will be patroling off San Diego and Seattle. Some 20 million Central American refugees will be flooding to our borders, their ranks swelled by "spies, saboteurs, terrorists and assassins." Before you know it, the U.S. Rapid Deployment Force will be "stretched along the Rio Grande."

That is not exactly a soft sell. For such a godawful collection of impending calamities, you'd expect something pretty persuasive in the way of preventive measures, some statement of a realistic objective offering a reasonable chance of success. And that's precisely the administration's problem with Congress, because right at the point of saying something positive, the administration's whole sales pitch falls flat.

The administration denies any idea of overthrowing the Sandinista government by force (though Reagan can scarcely conceal his preference for that outcome). To state it as a goal would be to invite the almost impossible question of how, even with the U.S. help he wants to give, it could be achieved against Nicaraguan armed forces which Buchanan tells us are the most powerful ever assembled in Central America.

So what we get is a mushy formulation. The president tells us the contras wish nothing more than a chance to lay down their arms and negotiate with the Sandinistas -- but from a somewhat stronger position of military strength. And what do they want to negotiate? They want to "recover" (Reagan's word) the original revolution against Somoza, with its promises of pluralistic government and civil liberties. How? By somehow winding back the reel to the moment just before the bad guys strongarmed the good guys and turned to Marxist Leninism and the Soviet Union.

The trouble is, the bad guys have pretty well settled in. As the president conceded at a breakfast meeting with reporters the other day: "The communist takeover has already happened, and we have a totalitarian regime in power." So when was the last time a totalitarian regime abandoned its revolutionary beliefs and sat down to negotiate itself out of power by agreeing to free and fair elections? It doesn't happen; the Philippine model is of no use. And it defies belief that it will happen in Nicaragua.

Buchanan claims the strategic issue is a simple one: "Who wants Central America more -- the West or the Warsaw Pact?" This conveniently leaves out the possibility that the Central Americans might want Central America more and that they don't think that a superpower struggle by force of arms, spilling across their borders and threatening ever deeper intervention by both sides, is the way to keep it for themselves. The leaders in the neighboring lands that have the most to lose don't seem to see the same threat the administration is trumpeting. That no more makes them soft on communism than is the president's congressional opposition on the side of the communists, as Buchanan cheaply charges and even the president implies.