Why This National Emergency Is Unprecedented

Since Congress passed the National Emergencies Act in 1976, presidents have declared more than four dozen national emergencies. As of today, 31 of those emergencies are still active. Yet many experts still believe that President Trump's declaration of emergency on Friday is unprecedented. We talk with one historian who explains why this use of power could have serious long-term effects on our democracy.

Heather Cox Richardson, professor of history at Boston College and author of "To Make Men Free: A History of the Republican Party." She tweets @HC_Richardson.