In a republican primary field that at one time boasted more than a dozen candidates, former South Carolina Governor Nikki Haley and former President Donald Trump are the last ones standing. That means Trump's fire is concentrated on Haley — a daughter of Indian immigrants. And he's using that heritage to try to undermine Haley's candidacy, and stoke concern about her legitimacy for the presidency. For the record, that concern is unfounded – Haley, as the Constitution dictates, is a natural-born US citizen. NPR's Mary Louise Kelly and Senior Editor and Correspondent Domenico Montanaro dissect the reasons WHY Trump keeps returning to this particular political playbook. Email us at considerthis@npr.org