Something that is an interesting concept in geometry is a depressing commonplace in Washington. In geometry, a line can have length without breadth. In Washington, the argument about the MX missile is like that.

Most MX opponents are ardent for the arms-control "process," one sour fruit of which is . . . the MX. In 1972 the misbegotten ABM treaty banned defense of ICBMs, thereby making a necessity of what many analysts, then as now, considered a virtue: deterrence based on mutual vulnerability. SALT I, signed simultaneously, was permissive and porous. (For example, it limited but neglected to define "heavy" missiles.) So the Soviet buildup, unconstrained by SALT I but legitimized by it, soon made U.S. land- based ICBMs vulnerable to a disarming first strike.

The MX was supposed to cure the vulnerability of Minutemen ICBMs in fixed silos. But after a decade spent considering 34 basing modes, the Pentagon now proposes to put MX in "improved" silos. The Reagan administration could candidly admit that this might create a "use-'em-or-lose-'em" hair-trigger situation in a crisis, and could plausibly argue that this might deter the Soviets from provoking a crisis. Instead, the administration lamely argues that improved "hardening" makes silos invulnerable after all.

The Reagan administration is stuck with an MX that others wanted for reasons related to arms control. Arms controllers are not actually hostile to strategic rationality, if rationality is compatible with arms agreements. But first things -- agreements -- first. Arms controllers know that the way to get agreements is to agree to limit things that are easy to count. So SALT I limited the number of launchers. Result? Bigger launchers packed with more warheads -- more eggs in more vulnerable baskets, like MX.

MX will survive in Congress this year, and by next year will have become a jobs program and probably will be invulnerable (to Congress, not Soviet missiles). It will survive this year thanks to only one thing -- the arms control "process," in which MX is, the administration says, a "bargaining chip." Actually, it is a bargaining chip between the administration and Congress, which supports it lest U.S. negotiators be weakened.

The president says the MX vote is "a vote on Geneva." By "Geneva" he means the arms control "process," during which, since SALT I, the number of nuclear warheads has quadrupled and the Soviets have deployed 21 new nuclear weapons systems. So, although it is fitting that Congress supports a new missile in order to sustain the arms control "process," it is dismal that, to sell this misbegotten missile, Reagan has become a zealous worshipper at the barren altar of arms control.

He says Moscow considers the MX vote a test of U.S. "resolve." But a nation driven from Lebanon by a truck bomb cannot restore its reputation by buying a missile for which three administrations have failed to find an adequate basing mode. A nation that lacks the resolve to use its ships to quarantine a regime such as Nicaragua's or its rifles to overthrow that regime, or even its dollars adequately to support Nicaraguan freedom fighters, cannot show relevant "resolve" by buying high-tech hardware. The West is losing World War III in small wars, not big-missile competitions.

Perhaps the president means that Moscow considers the MX vote a test of U.S. "resolve" to spend for defense. But Moscow would be distraught were Congress to spend the MX dollars on rifles and ships we might actually use, and on aid for freedom fighters in Nicaragua, Cambodia, Vietnam and Afghanistan. Congress could use the small change to buy better radio transmitters to override Soviet jamming that violates the Helsinki accords, which, speaking of resolve, the administration lacks the resolve to repudiate as dead letters.

But there is the rub and the reason that it is sensible to hold your nose, grit your teeth and support MX. If Congress kills MX, it will use the dollars to solve a different vulnerability crisis -- the vulnerability of Congress to constituents an spending cuts.

The current round of arms controlling in Geneva may last a generation. Or perhaps now that the kids have seized control of the Kremlin, the pace will quicken, in which case the two sides can reach a deadlock quickly. The only certainty is that the "process" will have produced MX, a missile conceived as a result of SALT I, gestated during SALT II and born in the hope of SALT III. Actually, the MX argument resembles not geometry, which is reasonable, but modern art -- say, abstract expressionism, which is the work of the confused, sold to the earnest.