So-called sanctuary city policies are a “public nightmare” that make American cities more dangerous, Rep. James Comer said Wednesday, opening a congressional hearing with mayors from four major cities the Trump administration says are hindering its mass deportation agenda and protecting people who are in the United States illegally.

The committee has invited the mayors of Boston, Chicago, Denver and New York City, all Democrats, to explain and defend the laws during a televised hearing at 10 a.m. Some mayors have defended the laws as making all their residents safer.