THE CONGRESSIONAL committees have completed their assigned budget cuts, on time and approximately on target. This weekend marks an astonishing moment in American politics. It's not only the scale of spending that's being reordered, but also the underlying law that authorizes and defines just about every kind of federal program. The Republicans have been doing some grumbling about slippery accounting, but you mustn't be misled. Most of these spending cuts are real and -- you need to pay special attention to this point -- they are being made not only for the 1982 budget, but permanently.

Friday was the deadline by which the legislative committees had to report the cuts imposed on them by the budget resolution that Congress voted last month. Stacks of bills have now been delivered to the Congressional Budget Office for analysis and tabulation. The CBO is to report its figures back to the House and Senate Budget Committees on Monday. Each committee will then pull all of this legislation into one gigantic reconciliation bill.

The reconciliation bill will run, by present estimate, to 3,000 or 4,000 pages. Each will contain many hundreds of substantial changes in federal law and policy. Since these are amendments to the basic authorizing legislation, they will also set the direction for the budgets after 1982.

The Senate, with its Republican majority, has generally amended the authorizations in ways that would continue to draw spending limits tighter in 1983 and beyond. The Democrats in the House have frequently done the opposite, to achieve the mandatory cuts next year but relax thereafter. The two very difficult versions of the reconciliation bill are to be resolved in a hugely complex and chaotic conference committee composed of nearly the whole Congress.

Or maybe not. The awful complexity of that conference, and the high rise of stalemate and breakdown, are obvious to the Democratic leadership. Perhaps it would be better, the argument goes, to let the conservative Democrats in the House vote once again with the Republican minority, as they did on the original budget resolution, and pass the White House's version of the reconciliation bill. Then the results will belong unambiguously to Mr. Reagan.

For the Republicans, the post-1982 effects of this bill are immensely important. The more it tightens spending authorizations for the later years, the fewer additional cuts the administration will have to advance separately, next year and the year after, when enthusiasm for the job may be waning and the next election will be coming closer.

This whole procedure is dangerous, since the reconciliation bill will contain far more policy changes than anyone is likely to be able to comprehend fully. Yet, as a practical matter, there is no other way to impose so sharp a turn in the evolution of the federal budget. There's more at stake in this summer's budget bills than mere money. They attempt to set new and sharper limits on basic American definitions of public responsibility.