An acute political interest attaches to the 17-4 vote in the Senate Budget Committee, which is controlled by President Reagan's own party, to cut in half the 10 percent rate of increase in real military spending that he had requested and strenuously lobbied for. The counterpart committee in the Democratic-controlled House had already voted for no more than a four percent rate of increase. It appears Mr. Reagan is headed for a major defeat.

Respectful of his mandate, Congress gave the president almost everything extra he wanted for defense in his first two years, notwithstanding the impact on current budgets and future deficits. This year, however, Congress decided it was safe and reasonable and necessary to demand a more solid strategic rationale. It made a parallel new demand for assurance that the Pentagon would spend the money wisely. On both counts, strategy and management, Mr. Reagan has come up short.

The current picture, however, cannot be attributed entirely to administration failings. Congress has worked hard in the last few years, and it has generated and been hospitable to a considerable body of expertise. A serious and wide-ranging defense debate has taken place. The upshot is that the legislators have improved their capacity for a responsible defense role. It is not anti-defense "liberals" who are in the saddle, notwithstanding presidential attempts to brush off skeptics in those terms. In both parties and both houses, conservatives dominate the defense opposition.

It is wise to listen to Sen. Sam Nunn's warning that legislators tend both to debate weapons systems rather than strategy and to end up protecting the programs dearest to their constituents and cutting the wrong things. Similarly, it is wise to be wary of a meat-ax approach based simply on cutting the budget by a given percentage.

Congress is, nonetheless, increasingly receptive to strategic approaches to defense: to approaches that measure the requirements of national security as well as the resources available to meet them. We publish in Topic A today one such approach, in which William Kaufmann, a high-level consultant to defense secretaries from Robert McNamara to Harold Brown, suggests ways to buy more usable defense than would the Reagan budget, for less money.

Even without further increases in defense spending, the nation's arsenal would be expanding greatly as a consequence of various expansion votes of the last few years. The procurement budget has risen from $35 billion to $80 billion just since 1980. The budget committee's plan for 5 percent annual real growth means the nation would commit almost $1.7 trillion to defense over the next five years. The current focus on the rate of increase should not distract attention from the central fact that an immense buildup is already going on.