New Jersey’s high-stakes Democratic U.S. Senate primary kicked into high gear Saturday as U.S. Rep. Andy Kim scored a surprisingly big victory over First Lady Tammy Murphy in the first test of their fight for the seat long held by indicted fellow Democrat Robert Menendez. Kim, 41, a three-term congressman, won the backing of the Democratic Party in Monmouth County — Murphy’s home turf — during its convention in Long Branch as hundreds of local officials and rank-and-file committee members packed into an unseasonably warm ballroom about a mile from the ocean to endorse a candidate for the June …