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.

In the few days since he returned to the White House, President Donald Trump’s sweeping executive orders and mass pardons have shattered political and legal norms. But one order is in a category of its own.