The curious tale of the Midgetman missile offers another lesson in how strategic sanity yields to secondary pressures.

Midgetman was originally conceived to be a light, mobile single-warhead missile, an important step back from the menacing world of "first-strike" threats posed by heavy multi-warhead nuclear weapons.

But Midgetman is jeopardized by the Pentagon's obtuse bigger-bang-for-a-buck philosophy. At a recent House hearing, Undersecretary of Defense Donald Hicks explained that the United States could buy 170 new three-warhead missiles for the price of 500 Midgetmen -- and save $20 billion doing it.

Perhaps obscure rivalries among defense contractors explain this bizarre proposal. It is hard to explain otherwise. The abandonment of Midgetman (or its transformation into a much heavier mult-warhead missile) would make nonsense of the recent -- and eminently sane -- proposals of the Scowcroft Commission.

That body was created some three years ago to get President Reagan and the Pentagon out of a self-imposed jam. This administration wanted to push ahead with the MX super-missile (10 warheads). Yet for essentially political reasons, it had scrapped the original "basing mode" in underground silos in the Southwest. It was in the weird position of wanting to build a missile it didn't know how to deploy. Secretary Weinberger was reduced, absurdly, to talk of basing it on planes. Enter the Scowcroft Commission. It hatched a plausible compromise. Proceed with the MX, it advised, but only as an interim "modernization." Meanwhile, look ahead to an eventual dependence on a mobile single- warhead missile: Midgetman.

The logic of the idea was far from esoteric, as nuclear strategies go. Huge multi-warhead missiles (both MX and its Soviet counterparts) encourage "first-strike" scenarios. As many-eggs-in-one-basket weapons, they invite preemption. They also threaten preemption against the missiles on the other side. They are pushing both the United States and the U.S.S.R. toward perilous hair-trigger "launch on warning" war plans.

How closely first-strike theory approximates any conceivable military probability is debatable. But much of nuclear strategy is built on speculative war-gaming -- and must be, since, fortunately, we have so far avoided experiments with the real thing.

The key point, given the need to deal rationally with such dire matters, is that the world would be far safer if both sides moved from first-strike missiles back to the stable deterrence offered by mobile, single-warhead missiles. (Their mobility would assure invulnerability; their single warheads would not threaten preemption.)

Everyone, not only the luminaries on the Scowcroft Commission but many outside it (Henry Kissinger, for instance) thouht the idea was splendid. The Midgetman strategy was gratefully accepted and endorsed by the president.

What has happened to it? If the Scowcroft report was read at the Pentagon -- and it surely was -- its message has been lost in the usual contracting rivalries and engineering contests. Even if the change proposed by Undersecretary Hicks saved money, it would be a madly false economy.

It is true that the beautiful logic of a return to single- warhead missiles has eluded not only the Pentagon but, so far, the Kremlin also. The Soviets, mystifyingly, have denounced Midgetman as a first-strike weapon -- which is exactly what it is not supposed to be.

But obtuseness afar is less dangerous to the survival of the Midgetman idea than obtuseness at home. It seems the usual pattern for major transitions in nuclear- weapons strategy to begin here and eventually find their way to Moscow. This was true of the fatally misconceived "MIRVing" of missiles (equipping them with more than one warhead). If the logic of Midgetman is as plausible as it looks, it will eventually commend itself to the Soviet strategic planners as well.

But not if the idea is stifled at the Pentagon. Not if Congress lets itself be talked, even on grounds of economy, into building just another heavy missile. If Midgetman is abandoned, the best idea anyone has had in years for arresting the dangerous slide toward hair-trigger first-strike strategies will vanish with it.