A long-simmering fight over New Jersey’s unique primary ballot design reached a federal courtroom Monday in a case that has the potential to upend not only the state’s tense U.S. Senate race but Garden State politics in general. A judge in U.S. District Court in Trenton heard more than eight hours of oral arguments in a federal lawsuit U.S. Rep. Andy Kim filed seeking to throw out the state’s “county line” system, in which candidates endorsed by the county party are bracketed together on primary ballots, with opponents listed to the side. The setup has become a big issue as Kim runs against Fi…