In the budget it is expected to adopt today, Congress squeezes the projected deficit for next fiscal year down to $37.7 billion, but only by dint of some economic and spending assumptions so optimistic that even some of those voting aye don't believe them.

The result is that almost everyone expects Congress to be back within a few months having to vote for one of two things: a markedly higher deficit of further cuts in federal programs.

This manipulation of budget assumptions is in danger of becoming an annual game. Last year President Carter proposed and Congress adopted with much fanfare a resolution projecting a balanced 1981 budget, a convenient fiction that was discarded almost immediately.

This year's suspect assumptions, like the Reagan administration forecast on which they are based, envision a rosy combination of rapid economic growth, swiftly declining inflation and much lower interest rates all occurring at once. Some of the assumptions could turn out to be correct, but it is highly unlikely they can all be achieved simultaneously.

A less favorable economic picture in 1982 could boost spending substantially in a variety of ways. Higher interest rates could raise the cost of financing the public debt by as much as $8 billion to $10 billion, for instance.

Many analysts think the $695.5 billion called for in the new resolution is unrealistically low. A better guess for outlays absent further program cuts would be somewhere between $715 billion and $720 billion, according to a diverse group of analysts ranging from staffers at the Congressional Budget Office and the budget committees to private forecasters such as economist Alan Greenspan of Townsend-Greenspan & Co.

Budget Committee Chairman James R. Jones (D-Okla.), whose version of the resolution was badly defeated in the House some weeks ago by an administration-backed substitute, is among the skeptics. When he brought the conference report on the budget resolution to the House floor yesterday, Jones declared flatly, "Fiscal 1982 will see a deficit not of $37 billion but of $50 billion or more. This is a mirage, a politically convenient mirage for the moment but one that may be less comfortable in the months ahead."

Projections in the resolution for later years, as with Reagan's projections, show a narrowly balanced budget in 1984. For Jones, that is part of the mirage. "The resolution relies on unspecified future reductions [in spending] to balance the budget. Of course, unspecified future reductions are the easiest to make, but I question whether we are being honest with the American people by taking this route," he said.

Jones noted that the budget Congress approved declined to count $3 billion that will be spent for oil to fill the nation's strategic petroleum reserve. And the budget's "interest rate assumption is almost certainly wrong by a very wide margin."

The Reagan administration has already run into an interest rate buzz saw this fiscal year. "Given the spurt in interest rates we are quite clearly not going to achieve our spending target," Murray L. Weidenbaum, chairman of the Council of Economic Advisers, told reporters yesterday. As a result, the administration is conducting "a bottom-up review . . . a program-by-program review" in an effort to find other program cuts to offset the higher interest costs, he said.

That's exactly the sort of exercise Congress and the administration will almost surely have to conduct for 1982 as well. The assumption that the interest rate on 91-day Treasury bills will average only 10.5 percent in 1982 is probably the most fragile part of the budget resolution's forecast.

Its fagility is underscored by the way in which that particular rate was picked. The administration forecast calls for an 8.9 percent average rate for 1982, and the House version of the resolution followed suit. The Senate Budget Committee said 12 percent is a more realistic figure. The conferees then simply split the difference in a decision that had much more to do with congressional comity than the future economy.

Each percentage point increase in rates can increase interest costs during a fiscal year by between $2.6 billion and $4 billion, depending upon how far ahead of the beginning of the year the higher level of rates takes effect.

With the prospects for the 1981 deficit now beyond $60 billion and heading for $70 billion, and that for 1982 somewhere between $50 billion and $60 billion, Congress and the administration may find it convenient to delay proposed July 1 personal income tax cuts until Oct. 1 or even would trim both years' red ink by about $10 billion, the 1982 deficit would still be nearly $46 billion, Greenspan estimates.

So much for Congress' commitment to $37.7 billion.