On the day that civilian policemen can stop ordinary citizens on the street at random and demand ID without probable cause of a crime to do so, that is the end of the Fourth Amendment and America is done.
If local police agree to do this in the name of immigration security, which is historically and constitutionally a federal responsibility, they will hasten that day. If this Gestapo like work is going to be done, let the Feds provide the money and the people to do it.
Local police forces, their leaders and their unions need to refuse to involve themselves in immigration crackdowns. It is not their job. If they do agree to participate, they become lackeys of the FBI and ICE, no longer local and autonomous.
If you are a local cop, remember that you are local. Serve and protect your citizens. Leave the secret police duties to federal agencies. Leave the application of terror to federal agents. They love it. Just because you can’t stop them is no reason to join them.