THE NEW congressional exercise known as budget reconcilation has been, so far, an astonishing -- and unexpected -- success. It's an unprecedentedly rigorous enforcement of spending limits on the congressional committees. The enforcer is the bill that, on Thursday, went into a gigantic House-Senate conference. With 100 conferees, this is, to put it mildly, a highly participatory process. It is also an interesting answer to the accusation that Congress has become unmanageably fragmented and chaotic. As a demonstration of fiscal self-discipline, the performance is impressive.

The current budget procedure in Congress originated in the taunts of presidents, notably in the Johnson and Nixon years, that in matters of money Congress could never say no. The Constitution gave the control of fiscal policy to Congress but, in practice, it had long since slipped to the White House. Six years ago Congress began to retrieve it.

The central innovation of the Budget Act of 1974 was a pair of resolutions -- tentative in the spring, but binding in the fall -- limiting each year's spending and its deficit. The unanswered question was which committees would retreat when (as always happens) their demands for money added up to more than the limit. Last fall, in the House, the quarrel became intractable and the session ended with cutbacks unresolved.

The chairman of the House Budget Committee, Robert N. Giamo, then prosposed a courageous solution. The spring and fall budget resolutions have always included substotals corresponding to committees' jurisdictions. In the past, these subtotals were purely illustrative. This year, Mr. Giamo proposed making them binding from the beginning. There was fierce resistance to the idea, mainly from House committee chairmen who saw the idea as an erosion of their power. But carried forward by the momentum of anxieties over inflation and last spring's campaign to balance the budget, the reconciliation rule has held remarkably well.

Over the summer, a good many people have begun to discount this year's congressional budget as irelevant. Next winter there may be another president, the argument goes, and there will certainly be a bill to reduce taxes. The congressional budget will have to be revised in any case -- so why does the current pulling and hauling make any difference? The answer is that the spending cuts now being imposed are substantial and, regardless of tax legislation next year, they are not going to be reversed. The precedents now being established promise to continue this close control in the future.

It does not put the matter too grandly to say that the basic question here has been the ability of a democratic assembly to restrain its own spending and to enforce its own budget limits. The reconciliation bill has not yet been passed. But, to degree that seems to surprise Congress itself, the outlook is increasingly hopeful.