But Will They Love Him Tomorrow?
Barack Obama is so charismatic that his greatest challenge has been trying to play down expectations during his first Senate term; Obama's seeming perfection--as gifted orator, award-winning author and proven intellect who was first black president of Harvard Law Review--has become something of political marvel in itself, as Democrats survey landscape of potential presidential candidates and wonder if he is one to lead them back into favor after season of darkness; Obama seems to be balancing need to reduce unrealistic expectations with wish to retain his golden mystique; photos (M)