THE ETERNAL VERITIES in defense now seem to have a shelf life of about two months.

Take the super-accurate, super-powerful 10- warhead MX missile, designed to hit such sensitive Soviet targets as missiles in their silos, command posts and the like. In March, it seemed to us and many others that as flawed as the missile was, it was worth keeping in production in order to strengthen President Reagan's bargaining hand at the newly opened Geneva talks. This was the theory on which Congress authorized the production of 21 more such missiles.

Just last week, however, a substantially new MX formula swept through the Republican Senate. The administration, slow to realize the MX was probably being saved from total extinction by the Democratic House, fought the new formula fiercely but inally went along.

The president had already agreed to reduce his original bid for a total of 200 MX missiles to 100. The Senate imposed a new lid of 50 for so long as the missiles are deployed in the current basing mode -- in old Minuteman silos. In those fixed and vulnerable silos, argued Sen. Sam Nunn, zeroing in on what has always been acknowledged as the MX's gravest flaw, a president would be under terrible pressure to fire first in a crisis in order to make sure that his MX force hit its intended targets (Soviet missiles in their holes) and that it was not wiped out in a Soviet first strike. A powerful, accurate weapon that will be targeted first and must be fired first is, unavoidably, a first-strike weapon, one inconsistent with professed American strategy.

The Senate did not entirely ignore the need to keep cards in the president's Geneva hand: some additional MX missiles will be deployed and the producproduce others for spares and tests. Still, it is hard to imagine what is going to remove the 50 cap. For more than a decade, successive administrations have examined dozens of basing ideas and have yet to find one that meets the reasonable Nunn test. The practical effect of his amendment is likely to be to top off the MX program and accelerate the program for a new, small, single- warhead, land-based missile, Midgetman. Midgetman is mobile and can be hidden, qualities that let it be withheld in the first stage of an attack and that make it a second-strike weapon.

We think Sen. Nunn has done a useful and important thing. Evidence of continuing American arms programs has to be shown the Russians -- who have long had "MXs" -- to put them in a bargaining mood. The 50 MX missiles (500 warheads) are not inconsiderable. But moving from the vulnerable and therefore first-strike MX to the invulnerable and therefore second-strike Midgetman is a trend well worth accelerating.