DOOR TO DOOR Ricky Hall, an Obama volunteer, in Charlotte, N.C., last week.Credit...Chris Keane/Reuters Late last year Matthew Barzun, an official with the Obama campaign, called Craig Fox, a psychologist in Los Angeles, and invited him to a political planning meeting in Chicago, according to two people who attended the session. “He said, ‘Bring the whole group; let’s hear what you have to say,’ ”