THE REAGAN ADMINISTRATION keeps on investing more of its prestige in its commitment to undo the Sandinista regime in Nicaragua. President Reagan, on Friday, reiterated his call for support of "our brothers, these freedom fighters," the contras. Since, at the same time, he said he opposed sending American troops to Central America, it is not clear how he intends to make good on his threats if the contra operation runs out of gas; it is going mostly on fumes right now.

A very slight change in American policy is visible, however, as a result of Secretary of State George Shultz's hour-long meeting with Nicaraguan President Daniel Ortega yesterday in Uruguay. Mr. Shultz did not think much of Nicaragua's recent gestures of offering to ship home 100 Cuban military advisers, to stop bringing in new weapons systems and to release the Nicaraguan conscientious objector it had grabbed from the Costa Rican embassy in Managua -- a step that should allow the Contadora discussions to resume. Presumably Mr. Shultz took this tack in the hope that if the United States keeps the heat on, the Sandinistas will offer more concessions. But, again presumably, he must also know there are limits on the new heat the United States can put on.

The Shultz-Ortega meeting did restore the U.S.- Nicaraguan diplomatic contact that the United States broke off in January. "I don't know that anything much has changed," the secretary said afterward. He reported, however, that President Ortega had agreed with him that the Contadora process is the "center of negotiations." Let us crack that code: Nicaragua had striven, in direct talks in Manzanillo, Mexico, to normalize relations with the United States. The Reagan administration took this as an effort by Nicaragua to give itself a free hand at home and to avoid the binding obligations to its neighbors that the would-be Latin mediators of the Contadora group have been seeking. So the question now is whether the Sandinistas have been sufficiently impressed by the Reagan administration's tough line to look again at those obligations.

One set of those obligations comes down to leaving their neighbors alone. A second and, for the Sandinistas, more difficult set consists of opening up the Nicaraguan political system. It is the Sandinistas' refusal to open up fairly -- Arturo Cruz, leader of the democratic opposition, proposed a new opening just yesterday -- that put the contras into the field and led the administration to support them. It is the military aspect of that support that the president is now fighting to keep from being finally repudiated.

No realistic person will say that the United States, if it loses the contra option, can ensure the progress of the democratic enterprise in Nicaragua. Nor can a realistic person ignore that the military path has brought the Reagan administration to the verge of a deep embarrassment. It is precisely the disparity between the immense stakes that Mr. Reagan keeps insisting are involved and the rather modest and uncertain resources he has available to achieve them that puzzles even friendly observers of his policy.