The measure of the sagging of the Reagan administration's Central America policy is that the United States is having to deal with a greater credibility problem than is Nicaragua.

Things may change; one hopes they do. An approach to peace will require the Sandinistas to accept a political opening that will be painful and divisive for them and they may be sorely tempted to cheat. Nicaragua's credibility will be on the line.

The United States is being called on to change not the character of its system but merely an aspect of its foreign policy -- to test the possibility of softening, rather than ending, Sandinista rule. As distasteful as this may be for Reaganites, it should not be beyond a mature great power.

The administration, however, or some part of it, is being a bit petulant and childish. It started up its new engagement with regional diplomacy by getting rid of Philip Habib, the American diplomat best situated to help the president accomplish the purpose he now says he has embraced.

Got rid of him, moreover, for the wrong reason. Habib resigned ostensibly because of differences over diplomatic tactics. Actually he resigned because of pressure from the Republican right wing -- pressure to which stand-tall Ronald Reagan submitted without any evident recognition of the damage he was doing to his credibility.

The sequence leaves the president without the major personage he would ideally want to have on board in order to pursue any serious initiative. George Shultz has a full plate, and, by failing to get the president to keep Habib on, has lost an important battle. Shultz's Latin America aide, Elliott Abrams, lacks the requisite standing.

Beyond personalities lies a question to which even the best informed Latins are seriously seeking an answer. Queasy in the knowledge that their fate is at the mercy of American political whim, they ask whether the United States is now going to work with Costa Rican President Oscar Arias on the plan that the five Central American states accepted in Guatemala earlier this month, or whether the United States, by a too-rigid insistence on some of its own standards or simply by a subtle diminution of energy at key passages, is going to undermine it.

The version of the Arias plan accepted in Guatemala improved on earlier drafts but is not complete and requires much plugging of loopholes, especially on the security side. Certainly it is not self-implementing. And some part of the implementation will fall to government ministers who may be political rivals of their presidents and to bureaucrats or generals with their own agendas.

To overcome the inevitable stickiness, there must be a sustained high-level thrust in Washington: leadership. Instead, Reagan takes the heavy hitter, Habib, out of the lineup in the crucial inning, allows self-serving statements (George Bush's) and rear-guard battles (Caspar Weinberger's) from lieutenants so inclined, and conveys the impression to the hemisphere that he may be sabotaging the best hope of the decade.

Everyone taking part in this exercise wants two things out of it: the benefits if it succeeds, and the avoidance of blame if it fails. Reagan is doing less than he might to reap the benefits. His course serves the Nicaraguan interest in avoiding the blame. It also strengthens an unfortunate Latin tendency to take refuge from harsh dilemmas in gauziness and accusations against the United States.

Does Reagan really have to have it pointed out to him that the next vote on contra aid, if it comes to that, will depend considerably on what taste his diplomacy has left in the mouth of Congress? Inevitably, there is going to be a debate on who has been acting in good faith and who has not. A serious administration, even one convinced that the Arias plan is a sham, even one prepared to dump Central America on the Democrats or the next president, should be anticipating that debate.

An irony is at work here that some Reaganites will not like: to improve prospects for renewed contra aid, Reagan has to work hard on a Latin peace plan that removes the contras from the military arena and inserts them only uncertainly into the political arena. A pity that Reaganites did not think of that during the years of their gauziness when they were looking for a military way to make American policy prevail.