The financial industry is making clear it wants no part of Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s plan to force Mexico to pay for a border wall.

The GOP nominee says he would convince Mexico to cover the multibillion-dollar cost of his wall by barring Mexicans who are working in the U.S. illegally from sending money back home.

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But banks and other financial companies say they do not want to get involved on the front lines in a contentious immigration battle, arguing there are questions about how effectively such a policy could be implemented.

“It should not be for the banking system to be the police or the immigration task force,” said one banking lobbyist. “I don’t think it should be left to banks to report that someone has come in to remit money to whatever country, and it may or may not be legal. That’s not our role.”

Billions of dollars are sent electronically all over the world using the remittance system. But while Mexican President Enrique Peña Nieto says he told Trump personally on Thursday his country would never pay for the wall, Trump’s campaign is sticking with their plan.

Under the proposal detailed on Trump’s campaign website, the Republican would rely on a section of the Patriot Act to effectively force banks and money transfer companies to determine whether a person trying to send money abroad is a legal resident of the U.S.

That section of the law was written to crack down on terrorist financing and requires banks and other financial institutions to verify the identities of people opening accounts and doing business with them.

Trump has vowed on his first day in office, he would expand that requirement to include wire transfers and require that no foreign person living in the U.S. can send money outside the country without first proving their legal status.

The businessman predicts that Mexico will agree to pay for a border wall within a matter of days to keep money flowing over the border.

The Trump campaign has also floated increasing visa fees, cancelling visas, and establishing trade tariffs against Mexico as ways to pay for a border wall, but cutting off remittances is the centerpiece of his plan.

Banks contend that they do not have a method of establishing the legal status of a person in the U.S. A person can open an account or send money abroad simply by showing an Individual Taxpayer Identification Number, which is given out by the government to people lacking a Social Security number, including people who are in the U.S. illegally.

The banking lobbyist said Trump’s proposal is not yet being taken seriously by the industry because it is still several steps away from even beginning to look like a reality.

Others argue Trump’s plan could have economic consequences across the globe, as typically impoverished areas are suddenly cut off from critical funds sent by family and other associates working in the U.S.

“Trump’s plan is to do this on Day 1. You couldn’t do it on Day 1 unless you planned on cutting off the United States from sending money anywhere in the world,” said Aaron Klein, a fellow at the Brookings Institution. “U.S. banks don’t know whether you’re a citizen or not.”

Klein is also an unpaid member of an infrastructure finance working group for the campaign of Trump’s Democratic opponent, Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE.

Money sent to Mexico from foreign countries has accounted for roughly 2 percent of its economy for over a decade, according to the World Bank. But Mexico falls behind several other nations in terms of the total dollars sent from abroad. The $25 billion Mexico received in 2015 was dwarfed by the $69 billion India received, and the $64 billion sent to China. The Philippines were also a larger recipient of foreign funds, as people there were sent $28 billion.

However, the World Bank has also found that Latin America and the Caribbean are the fastest growing areas for remittances, with much of that money coming from workers in the United States.

In 2014, the Bureau of Economic Analysis estimated that $40 billion was sent out of the U.S. to foreign countries via remittances.

Trump’s idea is not entirely new for Washington. Sen. David Vitter David Bruce VitterLysol, Charmin keep new consumer brand group lobbyist busy during pandemic Bottom line Bottom line MORE (R-La.) introduced legislation in 2015 to implement a similar idea. Under his bill, anyone seeking to send money abroad would have to prove their legal status in the U.S., or else pay a fine equal to 7 percent of the money being sent. Those funds would then be handed over to regulators to cover the costs of that oversight, with the excess being spent on border security.

But since Vitter introduced in January 2015, that bill has yet to garner a single cosponsor.

Trump’s proposal has been dismissed out of hand by President Obama.

“The notion that we are going to track every Western Union bit of money that is being sent to Mexico, good luck with that,” Obama told reporters.

If Trump were to crack down on official wire transfers, some note there are other channels that could be used for remittances. For example, illegal immigrants could rely on legal residents to send money for them, or turn to digital tools that fall outside traditional banking. Sending money via cell phones would be another option.

Or, people trying to send money abroad could turn to more informal money transfer systems. For example, the “hawala” system has been used for hundreds of years to send money to parts of the Middle East and India, and relies on an informal honor system and a series of brokers. Similar informal arrangements could crop up in Latin America if Trump were to crack down on formal remittances.

And in other cases where the government has honed in on scrutinizing money transfers, banks have responded by getting out of the business altogether.

It is effectively impossible to send money to Somalia from the United States via a traditional remittance today, as banks have steadily stopped offering service to that war-torn country amid continued government scrutiny over where the money is ending up.