Debbie Wasserman Schultz, the chair of the Democratic National Committee, announced Sunday she is resigning her post amid controversy over her impartiality in the party’s primary process and leaked emails that showed her and top party staffers seeking to undermine the insurgent candidacy of Bernie Sanders. The email release early Friday, just days before the Democratic National Convention was set to begin, cast a pall over the celebrations that were meant to highlight the unification of the party after a bitter primary season, while setting back efforts to fully line up Sanders’ supporters behind presumptive nominee Hillary Clinton.