Trump's battle with sanctuary cities is the next phase of his confrontation with urban America

President Donald Trump's threats to relocate undocumented immigrants and asylum seekers to "sanctuary cities" capture his determination to constantly press the boundaries of law and morality with hardline immigration policies. But it may reveal even more about his political posture toward the nation's biggest population centers.

In 2016, Trump lost the largest metropolitan counties by a bigger margin than any Republican presidential candidate in modern times. In 2018, House Republicans were routed in suburban districts not only in metropolitan areas already trending toward Democrat -- such as the suburbs of...