President-Elect Donald Trump could get Mexico to pay for a wall along the southwest border of the United States and close a giant tax loophole at the same time, according to several experts.

Building the wall — and making Mexico pay for it — was a signature issue of Trump’s victorious presidential campaign. So critics pounced when he indicated earlier this month he intended to ask Congress to appropriate funds to begin construction. Broken promise, they chortled.

“Doing so would both fulfill a campaign promise and reduce the Tax Gap.”

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But the government cannot spend money without congressional authorization, and Trump has said Mexico will reimburse the Untied States for the cost. He has not laid out how that would happen, but the campaign floated a number of ideas, including using money seized from drug dealers and threatening to cut off money wired to Mexico by illegal immigrants living in the United States.

Last week, retired IRS officials Rob Warren offered the idea of cracking down on a loophole that sends billions of dollars to illegal immigrants working in the United States. Using the money for a wall would not exactly be the same as receiving a check from the Mexican government, but it could be a relatively pain-free way of paying for a wall that experts believe could cost $15 billion or more.

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Warren wrote in an online publication known as The Stream that the IRS estimates it collects $468 billion less each year than it should, a statistic known as the “tax gap.” He suggested that Trump stop allowing illegal immigrants from Mexico to claim child tax credits.

“Doing so would both fulfill a campaign promise and reduce the Tax Gap,” wrote Warren, who added that many of the dependents claimed by illegal immigrants do not even live in the United States.

The IRS issues an Individual Tax Identification Number (ITIN) to any worker in America who does not have a Social Security number. That helps illegal immigrants comply with the legal requirement that every worker pay taxes, whether they are doing so legally or not.

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Although Congress in 1996 specifically prohibited illegal immigrants from receiving other tax benefits, it is a gray area as to whether that prohibition applies to the Additional Child Tax Credit, which lawmakers created after the 1996 law. That tax credit is available to the working poor who pay too little in taxes to receive the full $1,000-per-child benefit from the normal child tax credit.

It is unclear how much money the federal government pays to illegal immigrants under the Additional Child Tax Credit program, but a 2011 report from the Treasury Department inspector general for tax administration suggests it could be substantial. That report found that the government paid $4.2 billion to 2.3 million people with ITINs in 2010. The vast majority of those workers were immigrants.

The Trump campaign referenced that report as early as August 2015.

“The IRS completely knows about it, acknowledged it, but there’s nothing in the code that lets them do anything about that,” said Chris Chmielenski, director of content and activism at the advocacy group Numbers USA. “That could be one way [to finance a wall], and it would make total and complete sense.”

Chmielenski noted that various bills have been introduced in recent years to require a Social Security number in order to claim an Additional Child Tax Credit.

But David North, a fellow at the Center for Immigration Studies, said new legislation is not even needed. The IRS has the authority to require Social Security numbers to receive the credit.

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“The policy problem is very simple,” he said. “That’s a very quick way of raising a whole lot of money … That money could be diverted to a wall, or more likely, a series of fences and some walls.”

Ira Mehlman, a spokesman for the Federation for American Immigration Reform, said the authority to build a barrier already exists under the Secure Fence Act of 2006. But Congress has never provided the funding. Mehlman said closing the tax loophole is a good idea.

“Whether it’s a way to make Mexico pay for the wall or not, it’s just good, common sense,” he said. “The treasury shouldn’t be writing checks to illegal aliens in the first place and shouldn’t be writing checks to illegal aliens living in a foreign country.”