U.S. Rep. Andy Kim filed a federal lawsuit Monday seeking to throw out New Jersey’s unique and controversial primary ballot design, months before he and First Lady Tammy Murphy face off in June for the Democratic nomination in the state’s contentious U.S. Senate race. Kim argues “the county line,” the nickname for the long-debated system, is unconstitutional and unfairly gives a big advantage to candidates who are bracketed together because they have been endorsed by county party leaders who hold tremendous power in Garden State politics. The suit says the line allowed voters to be “cynically …