A LOT OF reasonable people are having a tough time deciding whether the United States should resume military aid to the rebels in Nicaragua. Well represented in the swing group in Congress, these people do not want to give the regime a free ride, but they harbor serious doubts that aid to the contras is the best available way to blunt the Sandinista thrust. A presidential appeal that straightforwardly addressed the doubts in such people's minds might have drawn at least some of them across the line.

On television Sunday night, however, President Reagan took a different tack. It was not a speech pitched at people assumed still to be listening to policy debate. It was a speech aimed at bringing passionate and partisan opinion to bear on legislators perceived to be caught in a political bind. Mr. Reagan loaded an immense share of the whole future security of the United States on the vote coming up in the House on Thursday. A regional scene best depicted in shades of gray became in his treatment a stark and simple global drama.

In fact, the question is not whether the Sandinistas are communists of the Cuban or Soviet school. All that is now a given: it is true. The question is whether a foundering four-year American-launched intervention made up of troops of uneven quality and political complexion can somehow be given new life and made to project a promise that a substantial number of Nicaraguans will embrace as their own.

Mr. Reagan skipped past the issues of political viability, adherence to democracy, military performance and regional fragility that have forced the question of whether the contras are a wise investment. Many Americans, anticipating failure of such an investment, believe the United States might have to rescue it with its own troops. Nicaragua's near neighbors and the South American democracies believe they could do better trying to trap and mire the Sandinistas in the toils of a Latin regional negotiation. That is the reason they keep their distance from the cause he pleads in their behalf.

Politically, perhaps, the president may know just what he's doing. He may find the requisite number of Democrats fearful of being blamed next November for losing Nicaragua to communism. Even so, he does no help to the evolution of a sensible, sustainable policy by transforming what ought to be a careful damage-limitation strategy, pursued collectively by the hemisphere's democracies, into a lonely American crusade.