A federal judge Thursday blocked President Donald Trump’s executive order outlawing birthright citizenship —the policy automatically giving citizenship to anyone born in the U.S.—and declared it “blatantly unconstitutional,” as legal experts widely believe the president does not have the power to change the policy on his own.

A federal judge temporarily blocked President Trump’s order seeking to end birthright citizenship for the children of many migrants, a major hit to one of the president’s Day 1 orders.