Having a national ID and carrying it are two separate things. Many Americans already have a national ID (passport) and do not carry it.
Conversely, the most common state ID (driver's license) is effectively mandatory to carry at all times for most Americans because it is required to drive a motor vehicle and driving a motor vehicle is required to move around outside of major urban areas.
Conversely, the most common state ID (driver's license) is effectively mandatory to carry at all times for most Americans because it is required to drive a motor vehicle and driving a motor vehicle is required to move around outside of major urban areas.