President Carter's budget for fiscal 1981, which goes to Congress Jan. 28, is obsolete before it surfaces, a stillborn victim of the new cold war between the United States and the Soviet Union.

The assumption in this tense and anxious city is that whatever last-minute increases for defense have been cranked into the president's budget proposals, Congress will raise the ante, and take a close look as well at reviving the military draft.

In short, an incipient boom in defense spending is gathering force, perhaps further delaying the onset of that promised economic recession in 1980.

These are among the consequences of the Soviet invasion of Afghanistan, and even more worriesome possibility of Soviet intervention in Iran and Pakistan, which would threaten the Western World's supplies of oil from the Persian Gulf area.

On Wall Street, defense stocks already have perked up smartly. On Dec. 13, Defense Secretary Harold Wilson raised the commitment for real annual increases in defense spending over the next four years from 3 percent to 5 percent.

In addition, Carter has abandoned his opposition to holding back the world arms race, withdrawing a 1977 ban on the design and sale by American companies of war planes for eager foreign buyers.

Now, the renewed cold war is juicing things up even more. Washington Post military correspondent George C. Wilson writes: "Very good times indeed are around the corner for many defense contractors, large and small, if Carter's plans are approved by Congress."

Until these developments, the unofficial but reliable work from White House and Budget Bureau officials put expenditures for fiscal 1981 at $615 billion or so, with receipts of about $600 billion, based on a forecast for a modest recession in this calendar year.

But that spending figure looks too low to many experts, including Brookings Institution Research Director Joseph A. Pechman. In an interview, Pechman said that a $615 billion spending figure "doesn't reflect the implications of the heating up of the cold war."

Even with no shooting war, just getting defense units into position to respond to the Russian threat will add more money to the budget, Pechman said. "The real question is to what extent the expenditure numbers in th budget will reflect changes in military obligational authority," he added. The latter refers to monies for sophisticated equipment that may take a couple of years to produce.

The change in the picture is dramatic enough to raise the possibility that the much-promised 1980 recession will never happen or may be delayed. Pechman speculates that in contrast to the decline of 2 percent or more in real activity in the first six months of 1980, as generally forecast, the economy could show a further advance of 3 percent or 4 percent.

That sounds like a happy prospect. But a souped-up economy based on a bigger military budget almost would guarantee that a decline from the current 13 percent inflation rate -- one of the assumptions in the Carter budget -- also should be discarded. Wage and price controls would be that much closer.

At breakfast with reporters the other morning, liberal economist Arthur M. Okun suggested that over the next few years "there will be a real upward push on defense spending regardless of who is president." Okun didn't say so, but this is the clear probability even if the terrible risks of the moment recede due to some symbolic concession or withdrawal by the Soviet Union.

The mood in Washington from the president on down suggests that detente is dead for a long while, if not forever.

The costs of cold war revival are weighty, including subsidies to be paid by all citizens for bailing farmers out of their lost grain business with Russia.(As Okun notes, Carter didn't use the grain embargo as a way of lowering food prices at home.)

And, of course, heftier spending for guns ultimately must be at the expense of butter. If there is a hopeful note to be synthesized out of all this gloom, it is that, no matter how grim, a cold war is better than a hot one.