WHAT ABOUT a nuclear test ban? The treaty of 1963 banned explosions in the atmosphere, in space and underwater; repeated efforts to extend the treaty to underground tests have failed. Different administrations, including Jimmy Carter's, have determined that it was more urgent to level off or reduce existing weapons than to fashion a ban on testing that would head off new weapons. They have felt that some new weapons should be tested -- at least weapons, such as Midgetman, that serve the purpose of taking existing arsenals off a hair trigger. Verification has always been in contention. So has reliability testing: in a nuclear world, testing helps provide assurance that stockpiled weapons are accident-proof and that they work.

Nonetheless the test ban issue is back. It was nowhere near center stage at the Reagan-Gorbachev summit. There President Reagan was insistent on wanting to develop, and therefore test, some new weapons, including the model of his Strategic Defense Initiative, with a nuclear trigger. But the Soviet Union has been pushing it all the same.

On the American side, a ban plays to 1) people's desire to do something about a seemingly endless process of arms-building, and 2) fears that arms are controlling governments instead of the other way around. Many people understand that a test ban would leave both powers with large arsenals, arsenals more dangerous than they need be. Especially when other arms control progever, a market expands for an arms-control step that is, as Mikhail Gorbachev says, the "simplest" around.

Actually, on the Soviet side, other considerations appear to govern. Arguing for a ban, Mr. Gorbachev has stated that doubts that might grow about the reliability of one's nuclear arms, if they were not tested, could slow down a reach for the trigger in a crisis. This seem silly; no less implausible, considering its source, is the lament that further underground tests will "deface our beautiful planet."

His more serious calculation may be that the Soviets have more to lose than gain from a foot-to- the-floor technological and economic competition with the United States. If this is so, however, the Soviets can ease their burden by arms reductions. Moscow may also hope a ban will shut down SDI. Again, if this is so, Moscow can come in the front door in Geneva where space arms are already on the table; a test ban is the back door. Moscow may also calculate that a closed society can shave compliance or resume testing more easily than an open society. If this is so, it is no comfort to Americans.

Two weekends ago, Mr. Gorbachev offered Mr. Reagan what he said was a last chance to join the seven-month Soviet unilateral testing moratorium. He proposed an early one-issue summit in Europe to start negotiating a full treaty ban. The administration at once complained about being pressed in public. For rejecting the offer, it will pay a certain price in international and domestic opinion. But a bad weekend in the propaganda wars is not going to turn Mr. Reagan around. Congress, in the unlikely event that a majority was so disposed, could seek to enforce a moratorium by denying test funds, but it cannot compel him to negotiate a ban.

We do not say any of this cheerfully. Mr. Reagan had reason to say no to Mr. Gorbachev on a test ban, even though in saying no he is turning down the one arms control agreement within visible reach. He is saying, too -- otherwise, he could not be taken seriously -- that he can do better. Let him do so.