WHEN THE SENATE returns to work this week, it will have to face up to the fact that the much-heralded budget compromise reached between the president and the Senate leaders doesn't offer much promise. It's important, as reassurance to the financial markets, that the budget process didn't bog down entirely. But the details of the compromise make apparent why so few senators, including Republicans, currently support it.

The linchpin of the compromise is agreement by the president to cut the buying power of Social Security and other pension benefits over the next three years. That's not a good idea on any terms -- the Congressional Budget Office estimates that the plan would put about 600,000 old and disabled people into poverty. But, with the trust funds in reasonably secure shape, even a more modest curb on Social Security would be hard to justify unless it could be truly argued that the compromise called for broad sacrifices. In fact, the losses are concentrated on the same vulnerable groups that have already sacrificed most even as the deficit has soared.

Neither of the two big beneficiaries of the Reagan administration's fiscal policies are called upon for real sacrifice. All that is asked of the Pentagon is that it slow down the rate of increase in its budget. But the military is actually planning step-ups in its buying: The other day it revealed that the cost of 84 major weapons systems had jumped by another $25 billion, mostly because it now plans to buy more of each.

Nor will taxpayers who pocketed the benefits from the enormous three-year tax cuts be asked for a contribution. Using its still-aborning tax reform proposals as an excuse for inaction, the administration opposes even efforts to stem the growth in tax shelters and other loopholes.

The plan, howeveld accomplish many administration objectives that are not measured in dollars alone. Targeted for oblivion are the last vestiges of the War on Poverty -- the Job Corps, community action and legal service agencies, job programs for welfare mothers and displaced workers and economic development aid. The savings are relatively trivial. But they serve the conservative cause of dismantling federal programs that might arguably provide rallying points or a network of support for the economically disadvantaged and their liberal advocates.

Gone, too, or severely reduced, would be many of the programs through which the federal government has tried to help meet local needs. Revenue- sharing, support for passenger trains and operating subsidies for mass transit and public housing would all be eliminated. Aid for highways and the air traffic controller system would be cut. Federal civilian employment would suffer another big cut -- though it is clear that morefederal workers are needed for tax collection, drug enforcement, immigration control and policing of hazardous waste and other environmental contaminants. Medicaid help for institutionalized people and other needy persons would suffer another big cut.

Most of these are dubious proposals. They provide no sound basis for the serious deficit reduction program that is still needed.