Eighteen states and the ACLU filed lawsuits seeking to prevent President Trump from denying citizenship to children born in the U.S. to non-citizens.

Put simply, under Trump’s order, the children of most undocumented immigrants wouldn’t be US citizens, nor would the children of people in the country on student, work, or tourist visas. This is more than a reinterpretation of the 14th Amendment — it’s an attempt to do away with the jus soli principle that undergirds US citizenship.

This story was updated on Jan. 21 at 6:08 p.m. to reflect that the ACLU’s lawsuit was filed on Monday, not Tuesday. Hours after President Donald Trump issued an executive order Monday barring birthright citizenship to children of undocumented residents,