DEFENSE SECRETARY Caspar Weinberger is beginning work on next year's budget and the five-year plan of which each budget is a part, but this time Sen. Sam Nunn, the ranking Democrat on the Senate Armed Services Committee, is pressing him to do it differently. Mr. Nunn wants the secretary to think seriously about a future in which the heady growth of the early Reagan years gives way to stricter funding. The senator inserted in this year's defense bill a requirement that the department lay out for Congress this fall how it would allocate funds through 1991 with either 1) no budget growth beyond inflation or 2) an after- inflation growth rate of 3 percent a year.

The purpose is to inform Congress, but even more to force the exercise on the department. Mr. Nunn's view is that this year's defense budget cuts are not aberrations but clues to a shift in the national mood. In the first years of the Reagan buildup, the military budget grew at an annual after- inflation rate of over 7 percent. The president asked for an increase of similar magnitude this year. The Senate has voted for zero real growth instead, and the House for what might be called real zero growth -- not even an inflation allowance. The defense buildup has foundered on the budget deficit. Almost everyone in Congress now believes that the military has a plateau in its future, and the only question is how level.

Mr. Weinberger, however, does not have a history of easy accommodation to defense cuts. When Congress has cut his budget before, his response has been to assume that the cuts were temporary and would be made up the next year. He has refused to adjust his long-term spending targets downward, on the theory that it was poor tactics to yield any more, any earlier, than he had to.

That kind of aggressiveness may have served Mr. Weinberger well in the first term. The question is whether it can be sustained in the current climate. Assume that future budget growth rates will be high and, magically, you have the dollars both to run existing defense production lines at efficient speeds and to start up new ones. There is money enough to make new commitments on top of existing ones. Change the assumptions a few percentage points and the new commitments make little sense.

Mr. Nunn believes that the services are now moving toward new commitments that they will not be able to afford. A long list of "new starts" is in contemplation: a new light Army helicopter, a new Air Force transport plane, new Navy and Air Force tactical fighters, a new vertical take-off plane, a new attack submarine. Hundreds of billions of dollars are involved. In a limited budget, Mr. Weinberger would have to put some or all of these aside; he would have to choose. That is what Mr. Nunn is pressing him to do.