This is one of the many, many things that the Leftists want to do that would be worth seceding over.
Just think about it a little deeper for a minute. If a state or locality has no ability to enforce its own laws because it has no law enforcement of its own, then what is the point of that state or locality passing laws that are by definition unenforceable - because they now lack the capacity to enforce laws? And if they can't pass new laws and regulate their own areas as they see fit, then what is the point of having states and localities?
Federalizing the police does away with federalism itself. We may have "states" and "cities", but they will be so in name only and will no longer have any purpose.
Again, if the federal government under the Leftists tries to go this route, then it is time to secede.