A new villain has emerged. This time he is not a "Middle Eastern terrorist," or even a human being.

No, the fiend of the hour is a uniform. "What we're talking about," said radio personality G. Gordon Liddy on one of his recent shows, "is what happened when the brutal thugs of the Bureau of Alcohol, Tobacco and Firearms, in their wannabe ninja suits, went to serve the warrant against the fundamentalist Christians down there in Waco." A National Rifle Association mass mailing included this language: "jack-booted government thugs, federal agents wearing Nazi bucket helmets and black storm trooper uniforms to attack law-abiding citizens."

On "Nightline" Monday, the agenda was to discuss links between the NRA and citizen militias. But the uniform rhetoric swallowed up most of the show.

It's the most nefarious equation possible -- that the U.S. government has become the Third Reich. And it all hinges on a suit of clothes?

The NRA language is "an outrageous statement to be made about any U.S. law enforcement official," says ATF spokesman Jack Killorin. "I think it defames everyone who serves their government. . . . It is demagoguery of the highest order."

Whatever it is, it has worked. The symbol itself -- the uniform -- has begun to take on a significance almost entirely separate from the reality of who did what at Waco, Tex., Ruby Ridge, Idaho, and Oklahoma City.

In an age when a Midwestern street can be indistinguishable from a war zone, when the enemy without has been replaced by a vague enemy within, and when police and military functions are increasingly blurred, uniforms speak a highly ambiguous language.

First, there are the facts about the uniform worn by federal agents, which turns out to be many different uniforms. Killorin points out that the ATF is not a "uniformed service" and so employees often wear standard business attire. But he said that in some situations, as at Waco, agents opt for "work utilities" -- informal fatigue-type suits that are more comfortable, have many useful pockets and give agents a way of identifying one another in chaotic situations. The uniform does not include jackboots -- above-the-knee footwear worn by 17th- and 18th-century cavalry officers, as well as fisherman and fascists. And the ATF's helmets, far from being Nazi surplus, are the standard-issue Kevlar numbers worn by U.S. forces in the Persian Gulf War.

In fact, a newspaper ad taken out recently by the NRA, in which agents have "ATF" emblazoned on their uniforms and are wearing Nazi-esque helmets -- very much the frightening ninja space warriors -- turns out to be not a photo of ATF agents at all. "That picture was not us," Killorin says, because the whole scene was staged by the NRA. A spokesman for the organization confirms that the photo is of a "reenactment."

As if to emphasize the symbolic potency of official costumes, however, Killorin takes pains to point out that the utilities are blue -- sometimes very deep blue, but certainly not black -- and always clearly display the insignia of the ATF. In other words, the agency is not trying to look like some stealth-of-night commando unit ordered up by the Pentagon. When military uniforms appear in any domestic conflict, serious legal questions often follow.

"I think we have made a fair effort to make it clear that we are law enforcement officers, not the military," Killorin says.

Police uniforms, which are often blue, suggest that the wearer is engaged in protecting and serving the community. Militaristic uniforms, particularly when they are black, send signals that have more to do with authoritarian force.

And the problem is that for the lay citizen the two can be hard to distinguish. Do federal law enforcement agents on the most dangerous missions really look much different from soldiers on the battlefield? As even the ATF's Killorin points out, some of the gear is the same.

Take the photo that appeared in last week's Time magazine, of five FBI agents training at Giants Stadium for last year's World Cup games. Their uniforms are all black, including enormous gloves and a balaclava hood that covers all of the neck and head except for the eyes and nose, and they are holding enormous black weapons. As may be appropriate for such a mission, the image is extremely menacing -- the kind of thing one associates with Special Forces, Israeli commando units, the guards outside foreign government buildings.

"How disgraceful!" says art historian Anne Hollander, who writes books about the evolution and meaning of clothing, when the Time photo is described to her over the phone. "I think black is too bad under those circumstances, because it does on its own suggest unnecessary menace and Nazism."

But even black, arguably the most powerful of colors, carries multiple meanings. In Western culture it was for centuries the color of self-renunciation and piety. But since the Renaissance it has also come to connote a sinister power, as in the black-shirt uniforms of storm troopers.

"If you sit with your hands folded and don't say anything, you look virtuous wearing black," Hollander says. "But if you use a black mask and a black uniform shirt, you are impinging on an area of terror and fear."

According to costume experts, popular culture plays a crucial role, though often an unconscious one, in uniform design. Margaret Vining, a specialist in the military collection at the National Museum of American History, says the trend in military uniforms in recent years has been away from spit-and-polish formality and toward camouflage and other casual dress -- the very SWAT-team dress that has been adopted by the federal agencies. "They leave the impression of a stealth Darth Vader look-alike," she says. "I wonder if they were influenced by current entertainment." Killorin says there is no particular design aesthetic behind the ATF garb, and the FBI did not respond to several requests for an interview about its uniforms. "Intuitively, or on some unconscious level, whoever designed that {FBI} uniform is certainly tapping into associations between the color black and authority, invincibility, the power to violate laws with impunity," says Michael R. Solomon, a professor at Rutgers University who studies consumer behavior and the psychology of clothing.

Sometimes federal agents wear yellow windbreakers, Solomon notes, but not in situations where they want to project power. "If you're going to invade the compound at Waco, you don't want to appear benevolent and friendly."

The color of a uniform affects not only the watcher, but the wearer. Solomon cites a 1988 study of professional football and hockey teams that found that teams wearing black uniforms consistently were among the most aggressive, and ranked among the highest in penalties per season.

Just as uniforms borrow from popular culture, clothing designers borrow back from the uniform world, with ironic intent. Joan Kron, editor at large for Allure magazine and a longtime writer on fashion, says a designer like Jean-Paul Gaultier or Karl Lagerfeld is likely to seize upon the current debate, appropriate the look of those controversial outfits and in the process perhaps dilute some of their power.

"Fashion always co-opts these things," says Kron, "but they completely remove the meaning. I can imagine we'll see them coming down the runway in six months." CAPTION: The FBI's SWAT team in menacing protective garb training for World Cup security. CAPTION: Uniformed U.S. agents enter James Nichols's farmhouse in Decker, Mich., two days after the Oklahoma bombing.