The 2013 Gang of Eight comprehensive immigration reform bill is the signature achievement of Marco Rubio's four years and ten months in the U.S. Senate. Yet in the first four Republican presidential debates, in which Rubio has played an increasingly prominent role, he has not been asked even once about the specifics of the legislation. Despite that omission, it seems likely that if Rubio continues to rise in the GOP race, someone, somewhere will pay attention to his most important accomplishment. The 1,197-page Gang of Eight bill is so far-reaching, and at the same time so detailed, that it provides...