Sen. Ron Johnson is calling on Treasury Secretary Steve Mnuchin to review the IRS's failure to crack down on "fraudulent overpayments" of the child tax credit to illegal immigrants under IRS Commissioner John Koskinen.

In a letter to Mnuchin sent last week and made public Thursday, Johnson, who chairs the Committee on Homeland Security and Governmental Affairs, said the IRS is making overpayments to illegal immigrants who apply for the Additional Child Tax Credit, the refundable part of the child tax credit. Those payments are possible because applicants do not have to submit their Social Security number.

"Under the Obama administration ... the Internal Revenue Service (IRS) continued to provide a significant incentive for illegal immigration by failing to prevent the issuance of billions of dollars in fraudulent overpayments of the Additional Child Tax Credit to illegal aliens," Johnson wrote in the May 26 letter. He cited a 2011 report showing that $4.2 billion was paid out to people who not legally eligible to work in the United States.

"Despite the administration's prioritization of border security and calls from Republicans, Democrats, and an independent inspector general for the IRS to correct this problem, the committee learned in recent testimony that the IRS, under Commissioner John Koskinen, allows these fraudulent overpayments to persist," he wrote. Johnson asked the Treasury Department to review those payments for possible fraud.

For years, people have filed for ACTC payments with their Individual Taxpayer Identification Number, including many who are living in the U.S. illegally or are foreign workers. Most recently, the Treasury Inspector General for Tax Administration determined that the IRS paid out $3.4 billion to ITIN filers as of 2015.

"The IRS's failure to remedy this vulnerability on its own accord not only continues to cost taxpayers billions of dollars every year, but also provides an incentive for illegal immigration into the country," Johnson added.

The Wisconsin Republican asked the Treasury Department to brief the committee's staff on ACTC fraud by June 9.

Improper payments continue to be a problem for the IRS. According to the inspector general's report in early May, the rate of improper payments hovers around 25 percent for the earned income tax credit, the ACTC, and the American Opportunity tax credit, which benefits college students.

President Trump's budget called for major savings through new efforts to make sure illegal immigrants do not receive these credits.

Read Johnson's letter here: