The race to fill a vacant, safely blue House seat in New Jersey has set off an expensive competition between different interest groups, political figures and cash-flush super PACs to shape the future of the Democratic Party this year — one district at a time. The Democratic primary to succeed Gov. Mikie Sherrill in Congress has attracted 11 candidates from across the ideological spectrum and various levels of politics, including an ex-lieutenant governor, a former congressman, a grassroots