Arizona Sen. Jeff Flake gained notoriety in July 2016 when he challenged then-nominee Donald Trump head-on during a tense meeting with other Republican senators. Now, he has emerged as one of President Trump's most vocal Republican critics with his new book, “Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle." We'll talk with Sen. Flake about what he sees as President Trump's threat to conservative principles and democratic institutions.