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 a sharp picture of where Rubio would like to take the U.S. immigration system. Rubio has renounced parts of his own work, but it's not clear which parts, and it's not clear if he has renounced them for good or only until he determines they are more politically practicable.

So until Rubio faces the inevitable questioning about his work, here are some of features of the Gang of Eight legislation that might attract discussion as the Republican race goes forward.

1) More immigration. Comprehensive immigration reform means more immigrants coming to the United States, and with the Gang of Eight Rubio would have dramatically increased that number. "The legislation would loosen or eliminate annual limits on various categories of permanent and temporary immigration," the Congressional Budget Office wrote in its 2013 assessment of the legislation. "If [the bill] was enacted, CBO estimates, the U.S. population would be larger by about 10 million people in 2023 and by about 16 million people in 2033 than projected under current law."