THE START long-range nuclear weapons treaty is finally finished. But still lagging behind is another White House arms-control priority: a United Nations treaty to ban chemical weapons from the earth.Last May, President Bush called on chemical-weapons negotiators to settle major differences this year and ready a treaty for signing in May 1992. Little progress has been made in recent months, however, and a new United States plan that would ease some verification provisions is now roiling negotiation waters. "It will be very hard to meet the president's first deadline of finishing all outstanding issues this year," says Elisa Harris, a chemical-weapons expert at the Brookings Institution in Washington. Submitted in July, the new US proposal deals with a particularly sensitive aspect of verifying that countries are complying with a poison-gas ban: snap visits to sites suspected of harboring secret poison-weapon material. Agreeing on how to conduct these challenge inspections is perhaps the toughest issue still outstanding in the chemical talks. That doesn't mean they would be the only type of verification the treaty would authorize; others include routine surveillance of sites declared to have made or housed chemical weapons in the past. In 1984, the US proposed an unrestricted "anytime, anywhere" approach to challenge inspections. Since then US officials have had second thoughts. They now worry that such limitless access could allow other countries to spy on Stealth aircraft plants and other sensitive installations under the guise of looking for chemical weapons. Accordingly, in July the US proposed instead a softening of challenge inspections. Among other things, the new American plan would allow up to a week to pass before a country would have to comply with an inspection request. Access inside a suspected site could be refused, though "every reasonable effort," in the proposal's words, would have to be made to allay suspicions. The host country could offer alternatives to inside access such as surveillance overflights or observation platforms. "You can't keep them from looking at the area they've asked to look at. You can negotiate the activity they can conduct at that location," says a knowledgeable US official. This official argues that the proposal "is very carefully drawn" to balance US concerns about snooping with worry that rogue states could use challenge-inspection loopholes to hide chemical- weapon stocks. If a country refuses to let inspectors inside a particular warehouse "it goes into the report" and becomes a basis for evaluating whether cheating is taking place. Critics complain that the new proposal seriously weakens a crucial verification guard. It's no accident, they say, that among those nations favoring such a conservative approach to snap visits are China and some developing nations thought to have clandestine nuclear programs. The proposals "have been very poorly received by most of our allies," says Elisa Harris of Brookings. France has proposed giving the inspectors more power, for instance. The British are co-sponsors of the proposal, but they in fact favor a "managed access" approach that calls for shrouding sensitive objects or equipment at a suspect site, according to Harris. Though challenge inspections may be the primary outstanding issue at the chemical talks in Geneva, it's not the only one. Others include the structure of the treaty oversight council and the question of who would pay for treaty implementation. After a treaty goes into effect, nations that currently hold chemical-weapons stocks would, of course, be required to destroy them - and that fact might in itself be another obstacle to the pact's completion. That's because the Soviet Union is having a terrible time designing a destruction program. Technology is one issue, though the US has talked about sharing its own incineration techniques. More difficult to overcome will be political opposition. The central government in Moscow will have to gain approval of the Russian Republic for any disposal plan, since that's where most Soviet chemical weapons are stored. In the wake of the Chernobyl nuclear disaster, and considering the state of relations between Moscow and the republics, that approval may be difficult to obtain. Soviet citizens "don't have a lot of confidence in their government's ability to do this," Elisa Harris points out.