Donald Trump says he will force Mexico to pay for a border wall as president by threatening to cut off the flow of billions of dollars in payments that immigrants send home to the country, an idea that could decimate the Mexican economy and set up an unprecedented showdown between the United States and a key diplomatic ally.

In a two-page memo to The Washington Post, Trump outlined for the first time how he would seek to force Mexico to pay for his 1,000-mile border fence, which Trump has made a cornerstone of his presidential campaign and which has been repeatedly scoffed at by current and former Mexican leaders.

The proposal would jeopardize a stream of cash that many economists say is vital for Mexico’s struggling economy. But the feasibility of Trump’s plan is unclear both legally and politically, and also would test the bounds of a president’s executive powers in seeking to pressure another country.

In the memo, Trump said he would threaten to change a rule under the USA Patriot Act antiterrorism law to cut off a portion of the funds sent to Mexico through money transfers, commonly known as remittances. The threat would be withdrawn if Mexico made “a one-time payment of $5-10 billion” to pay for the border wall, he wrote.

“It’s an easy decision for Mexico,” Trump said in the memo, which was written on campaign stationery emblazoned with “TRUMP Make America Great Again.”

After the wall is funded, Trump wrote, transfer payments could continue “to flow into their country year after year.” He gave the memo to The Post in response to a written question provided to him before an interview last week.

Nearly $25 billion was sent home by Mexicans living abroad in 2015, mostly in the form of money transfers, according to the Mexican central bank. In his memo, Trump said that “the majority of that amount comes from illegal aliens.”

But that figure includes cash from around the world, not just the United States. In addition, a Government Accountability Office report in January said that it is difficult to track how much money illegal Mexican immigrants are sending versus those working legally in the United States.

Throughout the campaign, Trump has claimed that he could build his proposed U.S.-Mexico barrier for about $8 billion — a figure that numerous experts have described as dubious because of the costs and other obstacles to building a lengthy, impenetrable concrete barrier through numerous jurisdictions.

The odds of success for Trump’s proposal to pay for such a wall also are fraught with challenges. Although there is a shortcut in the Administrative Procedure Act that allows for “interim” regulations that take effect immediately without going through the regular public notice and comment process, there are limitations on that authority.

Based on the process for changes laid out in the Federal Register, Trump as president could potentially invoke a change by making the argument that illegal immigration is an emergency that must be addressed immediately or is a threat to public health or safety.

But such a rule would presumably apply to limiting wire transfers, canceling visas or raising visa fees — not about directly limiting immigration. That could make it harder for Trump to argue that any of those criteria meet the exceptions, according to some experts.

Trump: ‘The wall just got ten feet higher’

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Republican presidential candidate Donald Trump, when asked to comment on Mexico refusing to pay for a wall between the two countries, says, “The wall just got ten feet higher.” (Reuters)

After reviewing Trump’s proposal, one expert on immigration law said he is skeptical.

“Trump is giving an extremely broad definition of this section of the Patriot Act and what it allows, and it’d surely be litigated,” said Stuart Anderson, executive director of the National Foundation for American Policy, a nonpartisan think tank in Virginia. “It would be a large expansion beyond what the text reads.”

Anderson said Trump’s memo also leaves unaddressed how normal financial transactions across borders would be affected and whether there would be an overly aggressive federal intrusion into the growing number of financial transactions that take place over the Internet.

Entitled “Compelling Mexico to Pay for the Wall,” the memo is the latest attempt by the Republican presidential front-runner to offer more specifics about his proposal at a time when he faces tough head winds, including a potential loss Tuesday to Sen. Ted Cruz of Texas in Wisconsin’s Republican primary.

The memo includes rationales for a number of potential intimidation tactics, including increased trade tariffs, the cancellation of visas, and higher fees for border-crossing cards.