A TROUBLESOME confusion marks official policy on chemical weapons, whose first use the United States renounced in 1925 and whose production it renounced in 1969. The administration has struggled to persuade Congress to permit it to resume production of a new, safer-to-handle nerve gas. But it offers no consistent explanation why. Sometimes it emphasizes a need to deter Soviet use of chemical weapons in Europe. Sometimes it says it is creating a bargaining chip to move the long-stalled Geneva negotiations on banning the making, stockpiling and use of these weapons. Sometimes it says that deploying the new types in Europe will raise the nuclear threshold. These considerations are not inconsistent, but they spread all over the lot and convey the impression the administration will do anything to get the new gas.

Wisely, Congress has been slow to give its consent. It is not simply that the case for production does not stay put. The weapons carry an aura of horror and thrust an extra political burden on a Western defense structure that needs no more burdens. The Nixon decision to end production, moreover, gave the United States a bit of moral high ground that should not be casually yielded. Surely it helped the United States in its campaign to induce Moscow to stop using chemical agents in Indochina and Afghanistan that Americans are not in the business of making the deadly stuff themselves.

Still, the administration has kept pounding away. A year ago, Congress was able to slow down the official juggernaut only by requiring that the allies give their prior approval to production and join consultations about deployment. This is what the current argument is about. The administration engineered approval of production in a NATO military forum. Ten senators immediately complained to President Reagan that the legislation requires approval in a NATO political forum. The senators look picky. But the administration is asking for trouble, perhaps big trouble, by trying to bring a politically volatile new weapon into NATO by a side door. Moreover, the whole tricky question of deploying the new nerve gas -- on the territory of the one European ally that permits it, West Germany -- remains to be addressed.

The administration says innocently that it wants only to "modernize" an "aging" stockpile. Fortunately, there are other ways to ensure an effective defense. There are -- changing -- reasons of convenience but no overriding reasons of necessity for starting to manufacture nerve gas again.