WASHINGTON - Donald Trump proposed on Tuesday forcing Mexico to pay for his planned border wall by threatening to block remittances from illegal immigrants, which he said amounts to “welfare” for poor families in Mexico that their government does not provide.

The Republican presidential candidate’s campaign said in a memo that if elected in November, Trump would use a U.S. anti-terrorism law to cut off such money transfers unless Mexico made a one-time payment of $5 billion to $10 billion for the wall.

Trump’s pledge to build the wall has been a much-touted highlight of a platform targeting illegal immigration in the United States that has helped make him the front-runner to be the Republican nominee for the Nov. 8 election.

It is unclear how much a wall along the nearly 2,000-mile (3,200-km) U.S.-Mexico border would cost, and Mexico has been adamant it would not pay.

The memo elaborated on an idea Trump floated in August, when he suggested seizing all remittances tied to “illegal wages.”

It said that upon taking office a Trump administration would propose a rule mandating companies such as Western Union Co WU.N to require customers to prove they were legally in the United States. If Mexico agreed to fund the wall, Trump would drop the proposed rule, it said.

“It’s an easy decision for Mexico,” his campaign said, adding the country receives about $24 billion a year in remittances from Mexicans in the United States, most of them in the country illegally.

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“It (remittances) serves as de facto welfare for poor families in Mexico. There is no significant social safety net provided by the state in Mexico,” it said.

According to the World Bank Remittances project, flows from the U.S. to Mexico in 2014, the last full year for which it has data, were nearly $24 billion although it is unclear what portion comes from Mexicans living in the country illegally.

‘GOOD LUCK WITH THAT’

Democratic President Barack Obama called the remittance-blocking idea impractical and possibly self-defeating.

“The notion that we’re going to track every Western Union bit of money that’s being sent to Mexico, you know, good luck with that,” he told reporters. If Mexico’s economy collapses, it would just drive more immigrants to the United States, Obama added.

Mexican President Enrique Pena Nieto appeared to dismiss the proposal as campaign rhetoric.

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“The (Mexican) Presidency has no comment on any opinion made in the heat of the electoral process to choose candidates for the U.S. presidency,” the president’s office said in a text message to Reuters.

Any move to target payments sent home by people living in the United States could have a crushing financial effect in Mexico, the leading recipient of U.S. remittances.

Trump's proposal could also affect banks and companies that handle wire transfers, which also include MoneyGram International Inc MGI.O and PayPal Holdings Inc's PYPL.O Xoom.

The companies did not respond to requests for comment.

In addition to his wall proposal, Trump has accused Mexico of sending rapists and drug runners to the United States. Democrats and many Republicans have repeatedly condemned his comments as inflammatory, but his remarks have been enthusiastically received by his supporters, especially by white working-class voters.

In the memo, first reported by The Washington Post, Trump’s campaign repeated its pledge to target visas. It also cited imposing trade tariffs or enforcing existing trade rules as a way of forcing Mexico to pay.

Trump supporter Benjamin Proto, a Connecticut lawyer, acknowledged the remittance plan was unrealistic but praised the candidate for “looking at different ways to do things.”

The memo emerged as Republican candidate Ted Cruz appeared set to beat Trump in Wisconsin’s primary contest on Tuesday, a win he would hope would mark him as the best alternative to the New York billionaire.