THE MAIN business of this Congress is the budget, and under the strains of the past year, the process is changing. In the present lull, we offer a few observations by way of program notes:

There has been one major change that you need particularly to keep in mind. As the procedure was originally set up eight years ago, Congress was to pass every spring a first budget resolution that would be merely advisory to guide the committees in their summer's work. Then, when they had completed the years' fiscal legislation in September, Congress was to add up the figures and seal the totals into a second resolution that was legally binding. But as the struggles over the deficit have become more severe in the last several years, that second resolution has become harder to pass. It was never passed at all last fall. As a result, the authors of this year's first resolution for the fiscal year 1983 wrote an unprecedented provision into it: it automatically becomes the second resolution in late September, unless--and it's highly unlikely-- Congress should vote a different one. Without further action, the first resolution's spending ceilings and revenue floors and deficit limit will all become law and, through simply Parliamentary processes, enforceable.

But you also need to be aware that, for purposes of these legal limits, all spending and deficits are calculated as though the economy were expanding in line with the optimistic assumptions that Congress has attached to this budget resolution. Growth slower than the assumptions will mean an actual deficit higher than the one in the resolution. How much higher? It depends on the actual growth rate in 1983. That's why the deficit, even though set in law, will remain a matter conjecture.

In political terms, this whole intricate process is being steered by the senior Republicans in the Senate. It is conventional to speak of it as the Reagan budget, but, as it now stands, it's mainly the work of the Senate Republicans. Orderly completion of the work on this budget now depends on getting Mr. Reagan to support, explicitly and publicly, the crucial decisions that have been made in the Senate's Finance Committee.