LAW ENFORCEMENT authorities learned it the hard way from the National Rifle Association, and now it's happening to certain officials within the Reagan administration who wanted to outlaw the favorite firearm of tomorrow's traveling terrorist: the undetectable gun. They can tell you from close-up experience that if you turn your back just once on the NRA, you're targeted for a full-bore blast of lobbying by the strong political arm of the world's deadly weapons suppliers. Police officers all around the country are still furious at the NRA's opposition to waiting periods on handgun sales and its support of a misguided handgun-toters' law in Florida. And now the don't-give-a-millimeter NRA has succeeded, at least temporarily, in blowing away legislation that was being considered by the administration to ban so-called "plastic guns" and firearms that do not meet minimum standards of detectability.

The NRA's position has all the ingredients of a Bill for the Relief of Terrorists. The lobby has been killing off what had been support in the Justice Department and the Treasury Department for the plastic-gun ban. Last week, the attorney general suddenly withdrew the legislation from final consideration at the Office of Management and Budget. A spokesman at Justice said the bill was "put on hold" pending a meeting with the NRA and law enforcement groups to explore the possibility of a compromise. The police are understandably upset at the prospect of terrorists' finding it easier than ever to carry weapons past detectors at airports.

Some legislators on Capitol Hill agree and are sponsors of S.465, a measure that would ban these weapons. The bipartisan supporters include Sens. Strom Thurmond, Howard Metzenbaum, Edward Kennedy, Nancy Kassebaum, John Chafee and Chris Dodd. Police organizations -- men and women who are on the front lines and who are now doing their own lobbying to protect themselves as well as the public from free trafficking of handguns -- also are backing S.465. If the administration winds up tilting to the terrorists instead of the police, so be it. Members of Congress should see the dangers, stand up to the NRA and vote for safety.