Privacy
The American Civil Liberties Union (ACLU) says that a national ID card would be a "gravest of threats" to personal privacy. Others say that a national ID system could lead to surveillance and monitoring of citizens.
Civil rights
Some say that a national ID card system could threaten civil liberties and civil rights. For example, critics say that mandating national ID cards could violate the Fourth Amendment and right to privacy, or interfere with First Amendment rights to freedom of association.
Racial profiling
Some say that a system that allows police to stop people to check their ID cards could lead to racial profiling.
Costs
The Social Security Administration estimates that a national ID card system could cost American taxpayers at least $4 billion.
Replacing lost cards
If a national ID card is lost, misplaced, or stolen, it could be difficult to replace. This could involve long lines, repeat visits to the DMV, and potentially lead to arrest or detention.
Technical complexity
Other factors that could make a national ID system difficult to implement include migration, access to registration centers, and unprepared registration centers.