strong>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.

But Rubio still has not faced those Gang of Eight questions. And if he's not asked in Las Vegas, there won't be another chance until after the new year, when the next GOP debate takes place in Iowa. So, just in case Blitzer or his colleagues are inclined to bring up immigration Tuesday night, here are five questions Rubio might be asked:

1) Recently Pew Research asked Americans whether immigration should be "kept at its present level, increased, or decreased." Very small minorities — just 7 percent of Republicans, 17 percent of independents, and 20 percent of Democrats — support increasing immigration. Yet the Congressional Budget Office found that your Gang of Eight bill would have raised immigration levels significantly, increasing the U.S. population in the next decade by an estimated 10 million people over the increase that would already occur if current law remains unchanged. Why did you do that?

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