LAS VEGAS — It was widely remarked after the last Republican debate, on Nov. 10 in Milwaukee, that Marco Rubio has not been asked any question, in any of the first four debates, about the substance of his signature achievement, the Senate’s Gang of Eight comprehensive immigration reform bill.

The next debate is Tuesday here in Las Vegas. A lot as happened since Nov. 10 — Paris, San Bernardino, and a long list of controversial statements by Donald Trump. The moderator, CNN’s Wolf Blitzer, has said he’ll focus on national security.