As far as liberal financiers go, you don’t get much more powerful than S. Donald Sussman. Since 1989, the hedge-fund billionaire has pumped millions into the coffers of Democratic politicians and their political pet projects. Sussman sits on the board of the Center for American Progress, the Democracy Alliance and the dovish Israel Policy Forum, and he’s been one of the top contributors to left-leaning 527 organizations during the 2010 election cycle. But while Sussman has long kept a behind-the-scenes profile, a recent ethics controversy in Maine has flung him into the center of a complicated dispute over state residency,...