The protest on the Wednesday before Thanksgiving was called National Opt-Out Day, and its organizers urged air travelers to refuse the Transportation Security Administration's full-body scanning machines.

But many appeared to have opted out of opting out. The TSA reported that few of the 2 million people flying Wednesday chose pat-downs over the scanners, with few resulting delays.

There have been high-profile acts of civil disobedience in response to the two controversial procedures recently deployed by the TSA for primary screening - the body-scanning machines and the intrusive full-body pat-downs - including software programmer John Tyner's unforgettable warning to a TSA official: "If you touch my junk, I'll have you arrested." But the public seems less opposed to the scanners than civil libertarians had hoped. In a recent Washington Post-ABC News poll, only 32 percent of respondents said they objected to the full-body scans, although 50 percent were opposed to the pat-downs offered as an alternative.

That means opponents of the new measures will have to shift their efforts from the airports to the courts. One advocacy group, the Electronic Privacy Information Center, has already filed a lawsuit, calling the body scanners unconstitutional. Could this challenge succeed?

Courts evaluating airport-screening technology tend to give great deference to the government's national security interest in preventing terrorist attacks. But in this case, there's a strong argument that the TSA's measures violate the Fourth Amendment, which prohibits unreasonable searches and seizures.

Although the Supreme Court hasn't evaluated airport screening technology, lower courts have emphasized, as the U.S. Court of Appeals for the 9th Circuit ruled in 2007, that "a particular airport security screening search is constitutionally reasonable provided that it 'is no more extensive nor intensive than necessary, in the light of current technology, to detect the presence of weapons or explosives.' "

In a 2006 opinion for the U.S. Court of Appeals for the 3rd Circuit, then-Judge Samuel Alito stressed that screening procedures must be both "minimally intrusive" and "effective" - in other words, they must be "well-tailored to protect personal privacy," and they must deliver on their promise of discovering serious threats. Alito upheld the practices at an airport checkpoint where passengers were first screened with walk-through magnetometers and then, if they set off an alarm, with hand-held wands. He wrote that airport searches are reasonable if they escalate "in invasiveness only after a lower level of screening disclose[s] a reason to conduct a more probing search."

As currently used in U.S. airports, the new full-body scanners fail all of Alito's tests. First, as European regulators have recognized, they could be much less intrusive without sacrificing effectiveness. For example, Amsterdam's Schiphol Airport, the European airport that employs body-scanning machines most extensively, has incorporated crucial privacy and safety protections. Rejecting the "backscatter" machines used in the United States, which produce revealing images of the body and have raised concerns about radiation, the Dutch use scanners known as ProVision ATD, which employ radio waves with far lower frequencies than those used in common hand-held devices. If the software detects contraband or suspicious material under a passenger's clothing, it projects an outline of that area of the body onto a gender-neutral, blob-like human image, instead of generating a virtually naked image of the passenger. The passenger can then be taken aside for secondary screening.

TSA Administrator John Pistole acknowledged in recent testimony that these "blob" machines, as opposed to the "naked" machines, are the "next generation" of screening technology. His concern, he said, is that "there are currently a high rate of false positives on that technology, so we're working through that."

But courts might hold that, even with false positives, "blob" imaging technology that leads to a secondary pat-down is less invasive and more effective than imposing a choice between "naked" machines and intrusive pat-downs as primary screening for all passengers.

In the Netherlands, there's another crucial privacy protection: Images captured by the body scanners are neither stored nor transmitted. Unfortunately, the TSA required that the machines deployed in U.S. airports be capable of recording, storing and transmitting images when in "test" mode. The agency promised, after this capability was revealed by a Freedom of Information Act lawsuit filed by the Electronic Privacy Information Center, that the test mode isn't being used in airports. But other agencies have abused the storage capability of the machines. The U.S. Marshals Service admitted in August that it had saved more than 35,000 images from body scanners at the Orlando federal courthouse.

In evaluating the constitutionality of these scanners, U.S. courts might hold that the machines can't be considered "minimally invasive" as long as images can be stored and recorded.