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House Speaker Nancy Pelosi says a wall on the U.S.-Mexico border would be "immoral." Instead, she favors something she calls a "technological wall." Another top House Democrat, Rep. James Clyburn, calls it a "smart wall."

Instead of building an actual physical barrier of steel, concrete or some other material, Pelosi, Clyburn and other Democrats advocate employing an array of high-tech devices -- drones, infrared sensors, surveillance cameras and more -- to keep track of activity at the border without physical impediments to discourage illegal crossings.

"We cannot protect the border with concrete," Clyburn said recently. "We can protect the border using the technology that is available to us to wall off intrusions."

The problem is, a smart wall would not actually wall off intrusions. Indeed, the main feature of a smart wall -- in past debates it was often referred to as a virtual fence -- is that it will not stop anyone from crossing the border into the United States. It can detect illegal crossers and alert authorities to their presence. But it does nothing to keep them from entering the country.

That is especially important given the nature of the migrants crossing the border illegally today. In the past, many were single adult men who could be caught and quickly returned to Mexico. But now, according to the Department of Homeland Security, about two-thirds of the crossers are families and unaccompanied children, who by U.S. law cannot be quickly returned. Once in the United States, their asylum claims -- the vast majority are ultimately judged without merit -- take a long time to process. During that time, many simply disappear into the country.

The point, for those illegal immigrants, is not to enter the United States without being detected. It is to enter, be caught, and begin the asylum process that will allow them to stay, one way or the other.

A smart wall is no obstacle to such crossers. On the other hand, a physical barrier would be a big obstacle and, if placed in key areas of the border, would likely reduce illegal crossings significantly. That is precisely the kind of barrier that Pelosi, Clyburn and other Democrats oppose.

"The virtual fence does not actually block the entry of anyone like a real wall or fence does," said Jessica Vaughan, policy director at the Center for Immigration Studies, which favors tighter controls on immigration. "The virtual fence only works if there are enough Border Patrol agents around to arrest the illegal crossers who are detected.

"Most importantly, this technology would not help in the case of caravan migrants pushing through the obsolete and aging barriers we have in many places," Vaughan added. "And it does not help at all if the people who cross are detected only to be released after they state a fear of return, or because they brought a minor with them, or if they are unaccompanied minors led across by smugglers."

Given the nature of the illegal flow across the border, a virtual wall would be even less effective than it might have been in the past.

Still, Democrats insist it is what is needed. Rep. Scott Peters, who delivered the recent Democratic weekly address, discussed his home city of San Diego and called for "sensors and radar ... cameras mounted on drones ... (and) state-of-the-art technologies to detect tunnels."

"That is what real border security looks like, and I can tell you that San Diegans want that border security," Peters said. "But we do not want a wall."

Under Peters' plan, the Department of Homeland Security would have to rely on technology at the border; if it wanted to build a physical barrier, it would have to get special congressional permission.

Peters briefly acknowledged that San Diego has a border fence. What he did not tell listeners was that building that physical barrier saved the city from a crisis in the 1980s by dramatically reducing the flow of illegal immigrants. In 1986, with no barrier to speak of, an astonishing 600,000-plus people were caught trying to enter illegally.

Only after fencing went up in 1989, and was lengthened and strengthened a few years later, did the flow subside. People seeking to enter the U.S. illegally moved eastward, to points where there was no fence.

The border barrier was a key factor in solving San Diego's problem. Now, a congressman from San Diego is dead set against building a physical barrier to stop illegal crossings in other parts of the country the way it did in San Diego.

Peters reflects his party's leadership and their determination to stop the construction of any more barriers on the border. But Democrats from Pelosi down still want to appear strong on border security. Now, they are advocating sensors and drones and cameras that would watch an un-fenced border -- while doing nothing to stop the flow of illegal immigrants.