The lawsuit to block the president’s executive order is the first salvo in what is likely to be a long-running legal fight over immigration policy.

The 14th amendment starts off:

All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside

Which makes any attempt to make people born in the US not be citizens like this simply unconstitutional. Documentation of birth in the US is pretty much the only proof of citizenship that most Americans have, so getting rid of this puts every non-immigrant American’s citizenship in doubt.