Attorneys general from 22 states sued Tuesday to block President Donald Trump’s move to end a century-old immigration practice known as birthright citizenship guaranteeing that U.S.-born children are citizens regardless of their parents’ status.

Trump’s roughly 700-word executive order, issued late Monday, amounts to a fulfillment of something he’s talked about during the presidential campaign. But whether it succeeds is far from certain amid what is likely to be a lengthy legal battle over the president’s immigration policies.