Last August, Donald Trump said he would get Mexico to pay for a wall along the U.S.-Mexico border by, among other things, stopping Mexican nationals in the U.S. from sending money back to their relatives back home. Such an idea was widely mocked by immigration experts as both unworkable and ineffective.

Undaunted, Trump filled in more details about how it would work on Monday, in a memo first sent to the Washington Post. Essentially, he wants to use the $24 billion flow of remittances from the U.S. to Mexico every year as leverage. He would use an obscure section of the Patriot Act to propose a rule that would prevent money lenders like Western Union from executing wire transfers for anyone who cannot prove that they are legally in the United States. If Mexico refused to pay for the wall, he would implement the rule.

At first glance, that seems like a perfect dealmaker’s move: Use a big bargaining chip and a clever reading of a law to threaten your counterpart and force them to the table. But as policy it might actually be illegal—and experts on the huge remittance economy suggest it would have huge unintended consequences, the kind a real-estate dealmaker doesn’t have to think about, but a president does. If financial institutions stopped allowing remittances—which they might do on their own, if Trump spooked them with a potential Patriot Act violation—the money wouldn’t stop. It would just be driven underground, into the hands of money launderers and other criminals, boosting the fortunes of the exact people Trump wants to build a wall against.

“If you’re driving business away from Western Union, where’s that business going—and how do we track it?” said Dennis Lormel, who spent 28 years working as a special agent for the FBI and finished his career as chief of the agency’s financial crimes section.

Those aren’t the only questions it raises. We ran a few of them by experts.



Is it legal?

The first question, of course, is whether a president could do this at all. Section 326 of the Patriot Act requires American financial institutions to collect certain information on their customers so that the banks and, if needed, law enforcement can identify them. But money transfer companies like Western Union and their primary business model—wire transfers—were exempted from the rule because such restrictions were deemed overly burdensome for one-off transactions. Trump intends to eliminate both of those exemptions and “include in the proposed rule a requirement that no alien may wire money outside of the United States unless the alien first provides a document establishing his lawful presence in the United States.”

Would that be legal? It’s hard to say. Experts say the law give regulators wide breadth to implement the rule in whatever way they want. “But it’s not clear that something like that would withstand a court review,” said Timothy Ogden, the managing director of the Financial Access Initiative at NYU. He added that the courts could deem it unconstitutional, saying it violates property rights.

The legality might not matter, however, if it spooks the banks enough. Banks are already worried about facing regulators’ wrath for failing to abide by anti-money laundering rules; in recent years, such fears have led most U.S. banks to stop processing remittances to Somalia, where they can’t be sure money isn’t falling into the hands of bad actors, like terrorists and pirates. Regulators have responded by trying to clarify to banks what actions are permissible, but banks have still largely not re-entered the market. The same could happen with Mexico.

“[Banks] would totally get out of that business,” said John Byrne, the executive vice president of the Association of Certified Anti-Money Laundering Specialists, “because there is no way to determine with any certainty whether a transaction is illegal, suspicious, or what have you.” He added, “From a compliance standpoint, this would be a nightmare.”

Whether money lenders like Western Union would get out of the business is a tougher choice, Byrne said, since wire transfers effectively make up money lenders’ entire business. (Western Union did not respond to a request for comment.) If the rule eventually was upheld on the courts, it could have devastating effects for the firms.

“He would probably kill the industry,” said Don Semesky, who spent 30 years with the IRS criminal investigation unit and another five years running the DEA’s Office of Financial Investigations. “They would lose more jobs than they’d save.”



Is it just Mexico?

Trump’s proposal, as it is written, is not limited to remittances to Mexico or for only Mexican nationals in the United States. Is that deliberate—does he want it to apply to immigrants across the board? We emailed Hope Hicks, his campaign’s communications manager, who responded to additional questions with “The plan speaks for itself.” Taken as its written then, the rule would apply to all people trying to remit money to any country. In 2012, remittances from the U.S. totaled $123 billion. Ogden said it would be hard to limit the rule to just Mexico, no matter how it was written, as people could remit the money to another country and then send it to Mexico to circumvent the U.S. law.

“The reasonable comparison to make is Cuba, because we do have something that is very specifically targeted at Cuba that the rest of the world ignores,” he said. “The net consequence is not that there are not dollars in Cuba and it’s not that there is not money from the United States going to Cuba. It’s just that that money is expensively going to Mexico, Canada or the UK first.”



Who really gains?

Security and money-laundering experts who watch the underground financial economy say that in all likelihood, such a rule would not stop the remittances: It would simply send them into the underground economy. Couriers or others will help facilitate money transfers across the border; Mexican nationals could also transfer money through a digital currency like Bitcoin.

“I guarantee you, before the wall has five bricks in it,” said Semesky, “there would be an informal black market.”

Either way, the result would be devastating for law enforcement: they’d lose a powerful tool to trace dubious money tied to drug-runners, terrorists, or human traffickers.

Trump’s wall-funding scheme proposes a number of other policies to pressure the Mexican government to pony up, including raising tariffs, cancelling visas and increasing the fees on visas. Essentially, he’d risk a trade war against our next-door neighbor to get it built, with consequences to our closely linked economies that economists say could send both countries into a recession. But all signs coming from Mexican officials are they will not pay for the wall under any circumstance. On Tuesday, Mexico named a new ambassador to the United States—who has experience “defending the interests of Mexico abroad.”

Of course, that could just be a bargaining position too—leaving Trump with two options: Follow through on his threat or get caught bluffing. If Trump’s bravado is any indication, he would not allow himself to get caught bluffing. He would attempt to stop remittances. And experts have come to the same conclusion now as they did back in August when he initially suggested blocking remittances as a wall-building negotiation tactic.

“It should be filed under ludicrous pipedreams,” said Ogden.

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