Democrats are not fearful, we just think that the Constitution of the United States of America already covers this and in particular, the provisions of the 14th Amendment makes any attempt by any state to enforce laws like this an unconstitutional act.
Amendment 14 - Citizenship Rights. Ratified 7/9/1868.
1. 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. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States...