SENTIMENTto extend the 27-year-old partial nuclear test ban ban treaty to underground tests is growing, notably in the Soviet Union but also in this country. A conference has been called at the United Nations in January to press the case for a comprehensive test ban. Two fresh arguments for a CTB circulate: that the Cold War is over and that the new nuclear arms-control priority of non-proliferation will flounder if testers keep flaunting their bombs.

The reduction in East-West tensions certainly requires a well-structured reduction and reshaping of arsenals. Still, the United States remains a nuclear power -- one with global interests and commitments. For some indefinite period it has reason to have bombs: to provide, at much lower levels, insurance, deterrence, and -- as now in the Gulf -- a nuclear umbrella for its foreign policy. As long as the United States retains bombs, it must have the safest and most efficient ones, it must be confident that they work and it must make sure deterrence remains credible. These are the purposes served by testing. Americans need not apologize for it.

But, the second objection goes, testing whets the nuclear appetites of others and diminishes testers' capacity to deny them their own bombs. These assertions cannot be dismissed summarily, but against them must be set other weighty considerations. One is the role of the United States as a stabilizing world force. Another is the fact that the countries beyond the First Five that have gone down the nuclear path -- think of India and Pakistan, Israel and Iraq -- did so not out of simple nuclear envy but out of powerful political passion. They could have been stopped if at all only by strong security arrangements addressing their driving passions.

By negotiation, Americans and Soviets have now closed loopholes in the underground test ban, eliminated medium-range missiles and they are nearing deep cuts in strategic arms. By separate economic and political retrenchment, they are otherwise moving to meet their treaty obligations to reduce the nuclear peril. Some non-nuclear countries and advocates suggest that the nuclear non-proliferation treaty should be allowed to expire (in 1995) as a form of pressure on Washington and Moscow to accept a comprehensive test ban. That's cutting off your nose to spite your face. Perhaps there will come a time when some nuclear powers drop off the list. Meanwhile, the focus must be on keeping others from getting on.