Eight days ago, Donald Trump floated the idea that Ted Cruz might cause Republicans some legal headaches if he won the GOP presidential nomination because he was born in Canada -- albeit to a mother who was and is a U.S. citizen. Today, Cruz's citizenship -- and the questions surrounding it -- is the single most dominant issue in the Republican race and has, without question, slowed Cruz's momentum in Iowa and nationally. It's a perfect case study in both how Trump attacks and how he's able to so effectively sidetrack his opponents.