FILE - In this Sept. 3, 2015 file photo, Democratic presidential candidate, Sen. Bernie Sanders, I-Vt., poses for a photo with supporters in Grinnell, Iowa. The Bernie Sanders phenomenon has been almost entirely driven by white voters. Now hes out to overcome hurdles with black voters who are still learning about him and could shape whether his insurgent campaign for the 2016 Democratic nomination can last. (AP Photo/Charlie Neibergall, File)