[null and void]

Maybe:

‘If cities defy Federal law, then they lose Federal funds.’

I don’t like the ‘should’. Not imperative enough. Maybe ‘shall’ or ‘will’?

And I want to explicitly target sanctuary cities

[Tax-chick]

Then how about: “If sanctuary cities defy Federal law, then they will lose Federal funds.”

What I don’t like about this is (a) that “sanctuary cities” and “defy Federal law” is redundant; and (b) that “will lose Federal funds” is a predictive statement about a future that is actually unknown. It is entirely possible that the status of sanctuary cities will continue as-is for a considerable length of time.