The decision of whom to allow into our country and on what terms is a matter of national sovereignty, and our Constitution gives the federal government the power to "establish an uniform Rule of Naturalization," Art. I Sec.8, cl. 4. Thus immigration laws and regulations are set at the federal level by Congress and by the President, not by individual states, cities, or towns. The Supreme Court just affirmed this in June in the case of Arizona v. United States.
However, the fact that states and cities can't regulate immigration does not mean that they all treat immigrants the same. Federal law provides that state and local law enforcement can "cooperate with the Attorney General in the identification, apprehension, detention or removal of aliens not lawfully present." 8 U.S.C. Sec. 1357(g)(10)(B). State and local governments go to great lengths to show themselves to be either the most or the least "cooperative" in the enforcement of immigration laws. Federal law also bars most immigrants from most public benefits, even from state funds, unless a state affirmatively passes legislation which makes immigrants eligible. States vary greatly in what benefits they will grant to immigrants.
At one extreme is Arizona, where a 2010 law bans state and local authorities from restricting enforcement of immigration laws, and Governor Jan Brewer has declared that her state will not issue Driver's Licenses to young people who are granted "deferred action" under the new program for childhood arrivals.
Just this week law enforcement officers in Arizona received permission from a federal judge to begin verifying the immigration status of any person they suspect is in the country illegally, provided that the verification is done only while enforcing other state or local laws. This is one provision of Arizona's 2010 law which has not yet been held unconstitutional. What effect such verification will have is hard to speculate. According to an article by MSNBC, federal immigration officials do not plan to pick up illegal immigrants detained by local law enforcement unless the immigrants meet ICE's existing priorities for removal or detention.
In allowing the immigration verification provision to go into force at all, the Supreme Court cited to limiting language in the statute which states that officers may not make decisions based on race, color, or national origin which would be unconstitutional. Whether Arizona will be able to enforce this law in a constitutional manner remains to be seen. On the very same day that Arizona law enforcement were given the go-ahead, the U.S. Department of Justice announced findings that the Sheriff's Office of Alamance County, North Carolina has engaged in discriminatory policing against the county's Latino drivers. Arizona officers can certainly expect scrutiny from the feds who will be on the lookout for similar discriminatory enforcement.
My hometown of Chicago, in Cook County, Illinois by contrast is notoriously uncooperative with federal immigration enforcement, and our state and local officials have been passing laws and ordinances to make it easier for even undocumented immigrants to build a life here.
It is more than a year since Cook County Board President Toni Preckwinkle declared that the Cook County Jails (which hold some 10,000 inmates on a given day) would no longer hold inmates on "immigration detainers" if they otherwise qualified to be let out on bail. Just this month, the City of Chicago jumped on the "no detainer" bandwagon also, amending the 1986 "Welcoming City Ordinance" to prohibit Chicago police from detaining undocumented immigrants except based on a criminal warrant or conviction of a serious felony.
The State of Illinois has also taken affirmative steps to allow immigrants to live and participate in society regardless of their immigration status. The state health insurance for children (All Kids) is open to all Illinois children regardless of immigration status. In 2011, the General Assembly passed the Illinois DREAM Act, which set up a special scholarship fund for children of immigrants and also opened up the state college savings programs to immigrant youth.