An important shift appears to have occurred in the defense debate. The administration, badly beaten up this year in Congress, has modified its goals. It has given up the idea, which it held until quite recently, that the defense budget can continue to grow by large percentages in the president's second term. The new line of defense, in both senses of the term, is that the time has come for steadier growth. The chosen rate is 3 percent per year, after allowing for inflation.

That 3 percent is the long-term rate projected in the congressional budget resolution adopted last month. In past years the administration has not considered itself bound by similar projections. Congress has tried to move the budget onto less ambitious paths, and the Pentagon has just come back for more. This year for the first time it appears instead to have yielded. The administration's most recent official estimates of spending for the next five years, published last week, are built around a 3 percent growth assumption. They envision military spending authority of $302.4 billion in fiscal 1986, which will begin Oct. 1, and $397.2 billion by 1990. In April the administration was still projecting that by 1990 military spending authority would be $488.1 billion.

At the Pentagon, meanwhile, Deputy Secretary William H. Taft IV has just taken the services through a preliminary exercise on the 1987 budget in which the guideline was also 3 percent. The services had to propose some cuts in previous procurement plans.

The new posture reflects more than a yielding to pressure. It has an important substantive side as well. What has never been clear about the buildup is how it would and ought to end. Many of the Pentagon's best friends, both in Congress and in the community of former defense officials, have been fearful the administration was taking the military on a costly roller-coaster ride, in which boom would be replaced by bust and the buildup would be badly disrupted. They have been urging the administration to work instead toward a softer landing. Under some duress, that is what it is now doing.

The Democrats are also trying to decide where to go next on defense. The House still has before it the conference report on the main defense bill for next fiscal year. The bill conforms to the budget resolution, which the House adopted just five weeks ago by a vote of 309 to 119. But it is also $10 billion over what the House had provided for defense in two earlier votes this year. For this and other reasons, some Democrats want to send the bill back to conference. The spending authority is already $20 billion under what the president originally requested, but still $10 billion over this year's level. The $10 billion is to keep the services even with inflation (the promised 3 percent real growth will then begin in fiscal 1987).

The pressuring Democrats argue that the Pentagon can safely be denied this cost-of-living increase in this tight a budget year. They and other Democrats seem also to think it is good politics to be for less for defense, but that is surely open to question: the anti- defense image the Democrats have acquired has indisputably hurt them in some quarters. And it needs also to be faced by them that "less" is not exactly a policy either. If the boom years of the Reagan buildup are now over, to what pattern of expenditures should they give way? The Democrats, no less than the administration, ought to have to answer that.