DETAILS OF the administration's January budget proposals keep leaking out in a steady patter. No plumber is called. Could that be an oversight? Or is it a well-calculated decision to lessen the impact of releasing a budget whose most damaging leak is in its internal logic.

In theory an administration likes to keep details of its budget planning secret until the completed product is unveiled late in January. The secrecy is intended to prevent the coalescing of opposition to proposed changes and so to preserve a president's options. In the Reagan administration, however, the gradual release of painful details in advance of the full budget transmittal has become a familiar pattern. This year the leakage was unusually heavy.

About a month ago the Office of Management and Budget saw fit to share with congressional leaders and budget committee members a set of tables outlining budget-cutting targets for domestic programs. In this town, that is the functional equivalent of distributing them on streetcorners. Since then information has continued to seep out providing further details on Medicare payment freezes, cutbacks in veterans' health benefits, restrictions on aid for college students and so on.

One purpose of these leaks might be to soften up the opposition. By initially proposing to wipe out or sharply reduce such popular programs as the Job Corps or supplemental feeding programs for low- weight infants, the president can later appear reasonable when he compromises on less drastic cuts. And some cuts that might seem unthinkable to Congress when first proposed may ultimately gain at least partial acceptance simply because they have become a familiar part of everyone's thinking.

But the leak strategy could serve a more important purpose. That is, accustoming financial markets to the unpleasant likelihood that the administration has neither plan nor intent to do anything serious about the looming budget deficits. Perhaps that should have been apparent all along. But during the campaign there was a tendency in business and financial circles to assume that, once the election was out of the way, the president would face up to the fact that his defense and taxation policies are incompatible with moving toward budget solvency. So far that hasn't turned out to be the case.

Financial markets don't like surprises. If the administration maintained a cloak of secrecy over its budget development, financial analysts might conclude that something serious was going on. When the budget is finally unveiled, a sharp sense of disappointment might set off the modern-day equivalent of a panic. Now the markets will have time to adjust their thinking to the idea that, if anything much is going to be done about the budget, Congress will apparently have to do it. Whatever you think of the administration's policies, you can't fault its public relations.