When President Obama addresses the country Tuesday night in Chicago, the place where he launched his political career, presidential scholars say he will face a unique moment, unlike perhaps any modern president before him: He will be giving this milestone address just as his signature legislative achievement is threatened to be undone. As a result, the opportunity also exists for a unique farewell address, a rhetorical device that has been used by many but not all presidents to recap their tenure, help solidify their legacy and address the future.