Entering the U.S. illegally is a crime as is overstaying a visa. Denver routinely makes requests to federal authorities for assistance, but I guess Denver only believes it's a one-way obligation.  I truly hope Denver receives the full benefit of their decision....good or bad.  Also, Trump is not demanding anything. He is carrying out what he ad every other federal official has sworn to do....uphold the  constitution of the United States.  Immigration laws have been codified and are constitutional.