THE CENTRAL AMERICAN peace plan is a long way from being turned into reality, but a lot of Central Americans, including democrats, want to extend the deadline, and the five heads of government who are about to meet in San Jose' will be under pressure to go that way. The risk is evident to everyone: drifting into a never-never land where pursuit of the plan becomes a cover for abandonment of the plan. Yet a Latin consensus favors holding to what has been gained and pressing for the extra steps that will make progress irreversible. No one wants to take responsibility for killing an initiative that remains, for all its failings, the region's best hope, and no one appears to have a better idea.

On compliance, no one should be too high and mighty. Among the signatories -- the contras are not among them; this was a plan made and signed by governments -- the Sandinista regime has made only small gestures toward the inscribed goals of peace and democracy, and meanwhile it has become known that after it publicly signed the peace plan, it secretly signed with Moscow for a huge military expansion. But the United States -- not a signatory but a guarantor -- is in plain violation of the requirement to end outside aid; so are Managua's patrons in Cuba and the Soviet Union. Honduras is in plain violation of its pledge to stop giving the contras sanctuary.

Nonetheless, a rough mutuality holds between Washington and Managua. Nicaragua has only partially opened its politics, but the United States has only partially closed down contra aid. That this latter result springs more from the insistence of Congress than the initiative of President Reagan is beside the point. The American political system is enforcing a policy that reflects a confirmed national ambivalence on this issue: this is a weakness and a strength at the same time.

Mr. Reagan has not shown that the big aid package he seeks, which is meant to carry the rebels safely into the next administration, would have a positive effect. But a little package keeping up pressure on the Sandinistas through the next phase of maneuvering makes sense. The purpose would be to key the American response to the Nicaraguan response. This is the approach called for in the peace plan's standard of ''simultaneity.'' There is always the danger it could leave each side standing pat demanding that the other move first, but there is a momentum still, and the five heads of government meeting in San Jose' have the opportunity to move the plan along.