WHILE CONGRESS still struggles with appropriations for 1983 -- the fiscal year already two months old--the administration is readying its budget plan for fiscal year 1984. This accounts for those barely muffled cries of pain and despair now emanating from the Office of Management and Budget and various departments.

Getting ready for the president's January budget message is always a contentious business. This year, however, there is real reason for anguish on all sides. Thanks to excessive tax cuts -- despite two years of arduous budget cutting--the departments face deficits in the range of $185 billion, even assuming a steady economic recovery. Cabinet members, now familiar with their programs and constituents, no longer docilely accept OMB direction. They know that most of the fat has already been eliminated and that what is left is protected by powerful congressional interests.

Social Security, veterans' benefits and the other big pension programs dominate the domestic budget and they are off-bounds for the budget-cutters. Slowing down the escalation in Medicare and Medicaid benefits means offending the powerful medical lobby or shifting costs to states or individuals in no shape to cover them.

Even if the remainder of the domestic budget were eliminated, it wouldn't come close to eliminating the expected deficits. And most of it couldn't be spared. Direct welfare programs for the poor have already absorbed the bulk of previous cuts, and simple decency prevents any further inroads into these benefits. Cutting back on other grants-in-aid programs would simply shift more burdens to states and localities already frantically cutting back services and raising taxes.

That's why there is an aura of desperation about some of the rumored cuts. Deep slashes in personnel for such agencies as the Social Security Administration, Environmental Protection Agency, Food and Drug Administration and other basic public health agencies aren't likely to get anywhere in Congress.

None of this bodes well for the administration's control over the budget next year. President Reagan's hold over Congress has already weakened. With deficits so large in the offing, it will be still harder to persuade Congress to give the time of day to various unpleasant cuts. But it may once again fall to Congress to construct a sensible package of tax and budget measures that ensures steady progress toward a balanced budget when recovery comes.