Within hours of his inauguration, President Donald Trump signed an executive order that would withhold U.S. citizenship from the children of some immigrants born in the United States, calling the order a “big one.” The executive order would end a right that the U.S. Constitution has guaranteed for more than 150 years.

Trump’s executive order, signed in January, seeks to deny citizenship to children who are born to people who are living in the U.S. illegally or temporarily. It is part of the hard-line immigration agenda of the president, who has called birthright citizenship a “magnet for illegal immigration.”