A recent New York Times article on Michelle Obama looks at her emerging role as a close adviser to the president as well as a compelling public advocate for his proposed policies. While the article compares the new First Lady to one of her predecessors, Hillary Rodham Clinton, it already seems to me that a better comparison may be to Eleanor Roosevelt, who didn't hold an official policy-making position but nonetheless was a forceful and effective public presence, especially on issues involving compassion and caring for the less fortunate members of society.
I would't want to push the comparison between Michelle Obama and Eleanor Roosevelt too far, though. For one thing, ER's political positions were often much more progressive than FDR's, and ER didn't hesitate to oppose her husband's policies in her own articles, books, and speeches. The Obamas seem much more closely aligned politically, and Michelle's early role has been as loyal supporter. Also, ER had already raised her children by the time she was First Lady, while Michelle Obama's role as White House mother evokes images of the Kennedy era. Still, I suspect that Eleanor Roosevelt's highly effective role as public crusader and behind-the-scenes counsel will be the best model for how Michelle Obama will exert her influence in this administration.