THE MX ISSUE is back. This is just what Congress intended when it approved the R&D money last spring. It meant to use the annual defense authorization and appropriation bills to ensure President Reagan's delivery on his pledges to negotiate seriously with the Kremlin and to move ahead on the new single-warhead missile. Now the first chunk of production money is up for a vote, and Congress is on the job.

On the MX itself, little has changed since the R&D debate. Even the missile's defenders acknowledge with Sen. Gary Hart, an opponent, that it is a "vulnerable, destabilizing, first-strike weapon"-- vulnerable for being fixed and targetable, destabilizing for needing to be fired early in a crisis so as not to be knocked out in its silo, first-strike for being accurate enough to knock out Soviet missiles in their silos. If that were all there was to it, the argument would have been over long ago.

The case for the MX, however, lies not in its qualities as a weapon but in its uses in an arms control negotiation. Of these "vulnerable, destabilizing, first-strike weapons" of which the United States currently has none, the Soviet Union has 638-- SS18s and SS19s. Other than the MX, what incentive is there for Moscow over time to shift the main leg of its strategic force toward invulnerable, stabilizing, second-strike weapons--something it has refused to do for the last, MX-less decade? In vain, one searches Rep. Patricia Schroeder's anti-MX article on the opposite page for some recognition of the bargaining value of just working on the missile --a value central to the pleas made for it by President Reagan and Andrei Sakharov.

Legislators on both sides of the issue ask if Mr. Reagan is serious about bargaining. He faces an uphill climb to earn broad confidence. But the effectiveness of congressional pressure should not be underestimated. On the radio last Saturday, for instance, he addressed the widespread impression that the 100 MXs he has requested constitute a floor: "If an agreement is reached which calls for deep reductions--which, of course, is our goal--the number of (MX) missiles could certainly be adjusted downward." He went on: "As opportunities permit, the U.S. position will continue to evolve."

The House is to vote on the MX today; the Senate is also debating it. In our view, there is a stark and insupportable inconsistency between cutting the production money and demanding that the president negotiate at START. The best course is to start production, bearing in mind that deployment is a four-year process that does not begin until 1986, and to keep a beady eye on the START talks at each step of the way.