The editorial "Building the Right Missile" {Dec. 7} missed the point entirely. The small ICBM, the Midgetman, was terminated by the Senate Appropriations Committee because it was too expensive and not cost-effective.

The Post's analysis of the strategic balance and the role of the ICBM was flawed. It ignored the stabilizing features of the triad of nuclear forces, which incorporates strategic bombers, sea-launched ballistic missiles and ICBMs.

To accept the editorial's contention, one must accept the logic that each leg of the triad must independently score superior marks on accuracy, security, reliability, credibility and survivability. In fact, the whole point of the triad is that the relative strengths and weaknesses of each element counterbalance and reinforce the others.

To accept the position of The Post, one must ignore the billions of dollars recently invested in improving the accuracy, the range and, hence, the survivability of our sea-based deterrent, the Trident II submarine and its new D-5 missile. One must ignore our multibillion-dollar investment in the Stealth aircraft, which will ensure bomber survivability into the 21st century. The United States has made major investments in ICBM reliability and accuracy with the MX missile, and with new rail-mobile basing we will substantially increase the survivability of our land-based deterrent as well.

So why, in this tightening budget environment, should we invest another $50 billion in the small ICBM? The Post asserts that such an expenditure "strengthens deterrence and stability" -- as if the small ICBM were the only option available.

I believe the real reason The Post has chosen to ignore the Trident II, the Stealth bomber and the rail garrison MX in posing its false choice on the Midgetman is because it has been sold a line -- a political line at that: "Buy the Midgetman and you undermine SDI. Kill the Midgetman and you are forced to buy SDI."

Nothing could be further from the truth. The only competition between the Midgetman and SDI is between scarce defense resources. Other than a general contribution to U.S. defense posture, their strategic rationales do not overlap.

The Midgetman represents the pinnacle of MAD theorizing, an inevitable, perpetual and -- oddly -- desirable mutual vulnerability to ballistic missiles. SDI and the new Reagan-Gorbachev arms-control approach reject this view and propose as an alternative a defense transition leading to the eventual elimination of ballistic missiles altogether.

For all these reasons, the Senate Appropriations Committee correctly terminated the small ICBM program this year before it wasted more scarce taxpayers' dollars. Given the pressures on the budget, the United States can ill afford to fund programs, however attractive, that are not cost-effective.

PETE WILSON U.S. Senator (R-Calif.) Washington