IT IS EVIDENT across the political landscape that some steam has run out of President Reagan's rearmament drive. He and his defense secretary remain committed to his first spending goals, but elsewhere in the administration and in much of Congress, dedication has slackened. Polls show the public is still wary of the Soviet threat, but less ready to support the expenditures the administration sees as essential to meet it.

The reasons for this turn are varied. No new international crisis has come along to salt the wound of humiliation opened by the Iran hostage-taking. In most minds, the martial law crackdown in Poland did not so much freshen the Western sense of danger as confirm a Kremlin intent to prevail in its claimed sphere. The MX debate proved to be, among other things, a window revealing the arbitrariness of much defense planning. Economic distress and the politics of economic distress, meanwhile, have double-whammied defense: made the Reagan levels seem unfair or at least politically questionable, and put on defense a burden to carry its share of the cuts needed to shave those huge looming deficits.

The upshot is that some, not all, of the momentum for accelerated defense spending has been lost. A new consensus appears to be forming: spending should increase, but at a slower, steadier and hence more politically sustainable and economically tolerable rate; the new rate can be pursued without damage to national security if new projects are chosen with greater care; and the new rate has its own welcome implications of foreign policy moderation. The figure representing this emerging consensus is 5 percent. Its most recent supporters include three well-known Democratic eminences, Robert McNamara, Cyrus Vance and McGeorge Bundy, and a retired admiral, who is no dove, Elmo Zumwalt.

A 5 percent defense increase after inflation every year for the next five years: it's not the Reagan 7-to-9 percent, but it's not hay. As many people may find it indefensibly high as find it unacceptably low. We find it a sensible figure for purposes of planning what weapons the government needs.

A 5 percent growth increment, itself worth more every year, would come on top of an already greatly expanded recurring base of expenditures and would help buy an awful lot of defense. It would make a contribution to economic recovery, by its impact on the deficit, of some $135 billion. It should enforce upon the Pentagon, unless it means to dare Congress to take knife in its own hand, a new and necessary degree of budget discipline. It would help the president, now in a defensive crouch, take command of an issue likely to dominate the political debate of the next two years.