THE LEADERSHIP in both houses of Congress faces a period of maximum peril over the next few months. The decisions it makes will affect not only the size and shape of what may be the most extensive changes in budget and tax laws ever enacted by a single Congress, but also the future of Congress as a major participant in setting the nation's economic policies.

The next test will come when the giant budget reconciliation bills reach the Senate and House floors as they are expected to this week. The procedural rules that are now being devised must strike a delicate balance. If they are too loose, alowing relatively freewheeling debate, the budget process could dissolve into chaos. If they are too tight, shutting out almost all chance for discussion or amendment, this will feed the already growing concern among members that the bedget process is depriving them of a real say in policy.

In the House, strategy centers on adopting a debate-governing rule that will keep the Democratic budget package relatively intact and head off an administration-backed substitute. This would leave many differneces with the Senate to be resolved in conference -- a back-alley operation not subject to further review -- but it would also keep alive some of the better budget options.

The Senage's procedures are governed by strict rules set in the Budget Act, and discipline is not expected to be much of a problem. The leadership, however, must decide how to unload a mass of extraneous legislation added to budget bill -- little things like rewriting basic housing legislation and allowing wider trucks on the nation's highways, that surely deserve some public airing in their own right.

The fight now is over the $35 billion or so in cuts that the budget resolution called upon the authorizing committees to write into program statutes. You need to keep in mind, however, that to meet its spending targets, the administration must find an additional $14 billion or more in savings next year. Some of this is to come from reduced agency operating funds and tighter program administration. But you don't come up with $14 billion by picking up paperclips and unscrewing light bulbs. A big chunk is almost sure to be sought from further reductions in Social Security benefits, another inflammatory matter that must be kept under careful control.

With fall will come new hazards. The whole budget issue will be reopened for the second budget resolution, the final word on income and outgo before the start of the fiscal year in October. By then some of the assumptions built into the present budget resolutions -- like a 10.5 percent average interest rate -- may strain the credulity of Congress and require either reconsideration of earlier tax and budget decisions or acceptance of a larger budget deficit.

The congressional leadership must surely be pardoned if all this unfolds with less than clock-like precisions. A showing of real competence and control is essential, however, if Congress is to continue to claim a major role in the process by which the nation's spending decisions are made.