Gov. Chris Christie of New Jersey said on Saturday that if he were elected president he would combat illegal immigration by creating a system to track foreign visitors the way FedEx tracks packages. “At any moment, FedEx can tell you where that package is. It’s on the truck. It’s at the station. It’s on the airplane,” Christie said. “Yet we let people come to this country with visas, and the minute they come in, we lose track of them. We need to have a system that tracks you from the moment you come in.”