YOU MIGHT THINK that the Pentagon,

straining to keep whole the large increments in defense spending favored by the Reagan administration, would welcome every bit of help it could get. That it would be especially sensitive to the counsel of those legislators who were urging major defense increases long before Ronald Reagan came to town. That it would be aware that a very large part of its effort to cut the budget in civilian sectors hinges on making a good showing of readiness to control costs in defense.

You might think that, and you would be wrong. Legislation is before Congress now to strengthen the hand of those in Congress--and more important, those in the Pentagon--who realize the devastating impact that repeated swollen cost overruns have on the military budget and on the inclination of Congress and the country to keep voting big new sums for defense. The bill was authored by Sen. Sam Nunn (D-Ga.), an uncompromising strong-defense advocate, and it passed the Senate 96 to 0. No one dared vote, as Sen. John Tower (R-Tex.), a reluctant supporter, put it, "against motherhood."

But motherhood of this sort is not universally respected in the Department of Defense. The secretary opposes the amendment, and, it seems, some of his aides are working in the House, which did not act on the proposal, to defeat it in the defense authorization conference going into its final phase today.

Come on, fellows. The Nunn amendment is no blind, know-nothing attack on defense from the political left. It is a modest, informed, helpful caution originating on the center and right. It simply provides a mechanism--an obligation to report overruns on major weapons systems to Congress-- that will require the Pentagon's civilian brass and the service secretaries to ride closer herd on the programs for which they are formally responsible. A skeptic might even say that all the amendment does is make the legislature a somewhat more open partner in a procurement system that is going to keep on generating overruns anyway in the absence of more basic reforms. A defense leadership that cannot learn to live with this friendly measure is only inviting more stringent and hostile controls.