I've previously discussed how Rudy Giuliani's scheme to "stop illegal immigration" (one of his Twelve Commitments) by requiring a "foreigners-only" ID card would either fail or would eventually become a national biometric ID.

Here's another way that his card would morph from being just for foreigners to being for citizens as well. Consider the following quote (link):

"You then have a tamper-proof ID card for all the people that are in the United States [1]. They have to get a tamper-proof ID card when they come in, or if they hear they have to get one."

Many of those who "come in" are going to eventually become citizens. What happens then? Do they get to magically throw away their biometric IDs? After having a track record of using a biometric ID as a non-citizen, it's going to be extremely difficult to wipe the record, and there will be a great deal of pressure to require that former non-citizen continue using their card.

[1] I'll assume he just misspoke and he doesn't mean "all the people that are in the United States", since he's made it clear that it would just be for foreigners in past statements. (Although, that could have been a slip).