Heres where we get to important legal point #1: being undocumented is not a crime. Its a civil violation. Undocumented immigrants have rights under the U.S. Constitution. And according to due process, the police cannot detain anyone who hasnt at least been suspected of a crime. If a police officer encounters someone walking down the street who turns out to be undocumented, they cannot arrest that person because that person has not committed a crime (ICE, however, can). Similarly, if the police arrest someone undocumented for example, someone suspected of committing a crime, who is then cleared, they must let that person go.
Important legal point #2: holding an immigrant past the point when they should be released, just so that ICE can pick them up, is unconstitutional. Multiple courts have said so, and immigrants can sue the police for unlawful holding.
Here we get back to the point of sanctuary cities: in a sanctuary city, the police will release an arrested immigrant after hes been cleared of charges, posted bail, or completed jail time for whatever he was arrested for. A non-sanctuary city will hold that person until ICE can come pick them up even though that extra holding is not constitutional.
Keep in mind that all of the above only applies if the undocumented person has not committed any serious crimes. If they have, the police can keep them in jail by filing charges. Or ICE can present the police with a warrant or other order from a judge, which will result in a hold until ICE can come by.
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"Even if that's not the case, I would prefer this bill fail, since I'd rather see a Democrat get this win instead" - Doom_Art