THE PRESIDENT'S plea for funds for the MX is a perfect example of what is wrong with defense procurement. For years, the experts have studied ways of basing this new mobile land-based intercontinental missile to make it at once essentially invulnerable and yet countable in an arms control agreement. Dozens of schemes have been sifted through and discarded. In frustration, the Senate not long ago refused to pony up the $1.5 billion the administration is asking to start producing the MX, instead ordering the president to come up with a final option by next December. Its clear and sensible message: no basing, no MX.

Well, there is still no agreed basing solution. The administration's last-gasp candidate, Dense Pack, does not seem to hearten even its partisans. Mr. Reagan, however, is now beseeching the House to put back the MX production money. He would base the new missile "temporarily" in old silos.

Why would the president want to build a boat in a bottle? A new super-accurate, land-based missile, one able to strike enemy missiles and other hard targets, is deemed so vital to future American deterrence that the embarrassing questions are being stifled. There are plenty of things to be said pro and con about building "counterforce" weapons. To put one in an old silo vulnerable to an adversary's counterforce weapons, however, costs you its essential attribute--the invulnerability that allows a president to ride out a first strike and retaliate. Dense Pack may be no better: it could leave the MX vulnerable and violate arms control obligations to boot.

Meanwhile, Mr. Reagan argues that a failure to fund the MX will take from his hand a crucial bargaining chip he is playing to induce the Soviets to reduce their strategic forces. The president is right: it will undercut some part of his START negotiating strategy. But the devaluation of the MX as a bargaining chip cannot fairly be blamed on Congress. Precisely on account of the basing difficulty, it was never a chip with much value. Mr. Reagan was taking a big risk by making it part of his bargaining hand. For as long as arms control talks have gone on, there has been an argument over the merit of building arms as bargaining chips. In this instance, it isn't even close.

One of Mr. Reagan's purposes in START is to move the principal deterrent of both sides to the all- but-invulnerable oceans. Going to sea is a safe and sensible policy, and the wonder is that Mr. Reagan did not long ago invoke it to make a virtue out of the necessity imposed by the Pentagon's failure to find a good MX basing scheme.