And so we have come again to a moment of testing for Marco Rubio. His very public reservations over Donald Trump’s nominee for secretary of state, Rex Tillerson, have quickly put the Florida senator back at the center of attention. But by stepping out and threatening to derail Tillerson’s nomination with a key vote on the Foreign Relations committee, Rubio has also raised old and persistent questions about his core character. One year ago, many would have predicted that Rubio would be in Washington this week preparing to be sworn in as the nation’s 45th president. The Republican presidential primary...