Khizr Khan, the father of a fallen Muslim U.S. soldier, is an immigration attorney whose specialty is a program that critics say allows immigrants to buy their way in the United States.

According to Khan’s website, he specializes in law related to the E-2 and EB-5 programs, which hands out green cards to foreign investors that put at least $500,000 into certain American firms. The investor can also get green cards for certain family members.

Khan says he helps his clients buy real estate in the U.S. as well as start businesses.

“The E-2 and EB-5 are two of the most notoriously abused visa categories that essentially allow wealthy foreigners to buy their way to U.S. residency, and possibly citizenship, with a relatively modest investment,” Jessica Vaughan, the policy director for the Center for Immigration Studies,” told The Washington Examiner.

“The EB-5 is literally a ‘citizenship for sale’ program in which a visa for a whole can be bought for as little $500,000,” Vaughan told The Examiner. “It is literally a ‘citizenship for sale’ program, and it’s an amazing deal. Compared to other countries, America is the Walmart of investor visa programs,” she added.

Khan criticized Donald Trump during the Democratic National Convention for the Republican presidential nominee’s stance on immigration from Muslim countries.

“Have you even read the United States Constitution? I will gladly lend you my copy,” Khan said.

The feud has lasted all weekend and into the new week.

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