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.

Attorneys general from 22 states including Massachusetts, New York, Connecticut and Vermont are suing, saying President Trump’s attempt to end birthright citizenship through executive order is illegal.