POLITICAL ACCIDENT puts a key weapons vote, on the Midgetman missile, on the Senate calendar just as Mikhail Gorbachev arrives. The vote makes plain that the nuclear universe in which the two great powers live will be shaped as much by decisions each makes separately about forces that are permitted as by decisions they may make together about forces that are banned. Making the right distinction between weapons that should be reduced and controlled and those that should not is the essence of sound policy.

The mobile single-warhead land-based Midgetman is meant to replace the obsolete, vulnerable silo-based Minuteman and perhaps to complement the new multiwarhead MX. Small and hard to target, Midgetman could be expected to survive a first strike. Confident that it was available for use later, the American command would not have to fire it off ''on warning'' -- before it knew what was going on. Hence it fits well a strategy based on deterring nuclear war, and it is a good weapon to firm up strategic stability as arms control takes the raw numbers down.

The Reagan administration, however, has equivocated on Midgetman. Under congressional pressure it has been developing this missile, but in the START talks it has sought to ban it and the matching Soviet SS25. Why ban weapons that strengthen deterrence and stability? Because, the argument goes, the Soviets are ahead in ''mobiles,'' because Soviet mobiles are hard to hit, and because it would be hard to verify just how many had been deployed. To the objection that existing stationary MX missiles and even planned garrison-based rail-mobile MX missiles are targets vulnerable to a Soviet first strike, the reply offered is to defend them with -- you guessed it -- SDI.

A Senate Appropriations subcommittee last week removed all funds to continue work on Midgetman. An administration that protests mightily when Congress cuts its other programs stood by. The issue goes to the full committee as soon as today.

Midgetman is central to the future of deterrence and stability; this consideration dissolves complaints that the Kremlin currently leads in mobiles (why not catch up?) and that mobiles are hard to hit (that's their advantage). Inspection precedents set in the missile agreement the two leaders are to sign brighten prospects that deployments of mobiles could be efficiently policed. These are powerful reasons why the Senate should put Midgetman back on track.