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.

A federal judge blocked the Trump administration from enforcing a contentious executive order ending birthright citizenship after certifying a challenge by the A.C.L.U. as a class action, effectively...