The Democratic mayors of Denver, Chicago, Boston, and New York City were grilled for hours over their so-called sanctuary city policies at a Congressional hearing Wednesday as President Donald Trump pushes his aggressive immigration agenda and threatens to punish cities that don’t comply.

Immigration law, the mayors said, is a federal responsibility, and the attempt to put that responsibility on local law enforcement makes communities distrust the police and others they may need to call for help. Trump’s crackdown, the mayors said, has terrified immigrants, many of them in the U.S. legally.