THE CAMPAIGN to cut the 1982 defense budget is gathering strength in Congress--but it's too late. Defense spending imposes very long lead times, and legislative changes now will have little effect on actual outlays in the fiscal year that has just begun. As Congress gloomily contemplates the arithmetic of the 1982 budget, a good many people there--both Republicans and Democrats--say that they want another big cut in defense to help bring spending down to Mr. Reagan's target. But at this point, it's no longer a matter of good policy or bad policy. It's impossible except through the kind of extreme and pinch-penny economizing--canceling training exercises, for example, to save gasoline-- that is both unwise and, in the longer run, more expensive.

Merely to continue past defense policy, as the last Congress had laid it out by the end of 1980, would cost $183.8 billion in 1982, according to the Congressional Budget Office. President Carter, before leaving office last January, proposed a slight increase to $184.4 billion. Mr. Reagan raised the defense budget to $188.1 billion and then, in the cuts that he announced last month, pulled it back down to $186.1 billion. The reason for this recitation is not to repeat the familiar point that Mr. Reagan is working rather close to his predecessor's figures. It is to suggest that the successive figures for this year, from two very different administrations, through changing conditions, have all been within a very narrow range. For the near future, there is never much flex in defense spending.

Mr. Reagan and his Defense Department will, in any case, have great difficulty staying within the limit they have now set. The CBO warns that spending is very likely to run about $5 billion over their 1982 budget. Most of this money will be paid under appropriations already voted and contracts already signed. For the 12 months immediately ahead, spending depends less on congressional will than on contractors' performance, and the CBO says that past experience indicates faster payouts than the White House expects.

But the corollary is, of course, that the time to address the spending levels for 1983 and 1984 is right now. The programs currently under debate will begin to show up in the spending levels in those years. An increase for defense is clearly necessary, but the precise dimensions and purposes of that increase are very much open to discussion.

At this point on the calendar, you would be justified in regarding congressional votes to cut fiscal 1982 defense spending as little more than gestures. They would be mere diversions from the real defense issues, the commitments for the second half of Mr. Reagan's term and the years beyond.