https://www.nytimes.com/2017/04/21/us/politics/sanctuary-city-justice-department.html
Interestingly enough, they are using a technique I generally deplore, "Do as we say or else we hold back funding" Generally you see it in things like Educational decrees. However I've always felt education is a local issue not a national one size fits all. I do approve of cracking down on Illegal immigration and sanctuary cities.
Education is both a national and local issue. E.g. math and science and English and history don't change based on what state or city you live in. Thus, neither should educational methods or standards.
Anyway....
The reality is that those states are not violating federal law, and it is probably impossible for the federal government to legally withhold funds from cities that refuse to do more than what is required by federal law.
• Per Printz v. United States, the federal government cannot force states to enforce federal law.
• Per National Federation of Independent Business v. Sebelius, the federal government can't threaten large funding cuts to coerce states into adopting federal policies.
• It's also unconstitutional for the federal government to add strings
after the law is written. If that were the case, the Obama administration could have pulled all sorts of federal funding from states that didn't fully cooperate with the ACA.
• Congress decides how to allocate funds for almost all of the relevant programs. Not the Executive branch, or Jeff Sessions.
• The jurisdictions in question are complying with federal law, they just aren't taking
additional steps not specified in the law, such as requiring law enforcement to ask about immigration status.
• It doesn't help that there is no formal definition of "sanctuary cities," meaning that the federal government can apply the restrictions arbitrarily
And again, court decisions have consequences. Many of the above arguments were pushed by conservatives who disliked certain federal policies, and used Constitutional features to block them. Removing those barriers (which is unlikely btw) erodes the powers of the states, and gives the federal government more power.
Thus, if the current administration succeeds in these efforts, it's going to be
lots of fun when the next Democratic President uses the same tactics to impose not just federal laws, but executive decrees right down to the state level.