Donald Trump launched his campaign for president with a harangue against illegal immigration and a promise to build a wall along the U.S.-Mexico border.

Yet, inexplicably, moderator Lester Holt did not ask a single question on the topic in a 90-minute debate.

“What? Not one question on immigration? I could have gone to bed early!”

The omission was glaring — and noticed by those who follow the issue closely.

“What? Not one question on immigration? I could have gone to bed early!” tweeted Mark Krikorian, president of the Center for Immigration Studies.

Conservative commentator Ann Coulter resorted to sarcasm.

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“Great that debate covered Trump’s taxes, income, father, ’72 lawsuit, remarks as a reality tv star … & didn’t waste any time on immigration,” she tweeted.

It is not clear why immigration did not rate a question in the debate. The word came up exactly twice — once when Trump mentioned his endorsement by the union representing Immigration and Customs Enforcement officers, and once when the Republican nominee said some street gangs are largely filled with illegal immigrants.

Presumably, voters — whether they favor more or less immigration — would have liked to hear the candidates expand on their ideas in an area that touches on law enforcement, national security, and the economy.

Here some questions Holt could have asked:

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1.) How would Trump pay for the border wall if Mexico balks at his demand? The campaign has toyed with a couple of ideas. One would be to threaten Mexico with a loss of funds sent home by immigrants living in the United States. Remittances last year totaled nearly $25 billion, representing a bigger part of the Mexican economy than the oil-and-gas industry or tourism. Another possibility under consideration involves seized drug assets. It is likely many voters have not heard about those ideas and would have benefited from a substantive discussion on the possible options.

2.) How is Trump’s wall proposal substantially different from the Secure Fence Act, which Democrat Hillary Clinton voted for in 2006 when she represented New York in the Senate? It called for a 700-mile physical barrier, although the project never was finished because Congress did not back the law with sufficient funding. Other than the funding mechanism and the length of the barrier, is there a significant difference? “I can’t think of anything, and she did definitely vote for it,” said Jessica Vaughan, director of policy studies at the Center for Immigration Studies.

3.) Why does Clinton have such an uncompromisingly pro-illegal immigrant stance today when her position has been much more nuanced in the past? In 2003, for instance, she sounded downright Trumpian when talking about the need to control illegal immigration. “I am, you know, adamantly against illegal immigrants,” she told WABC radio.

4.) What does Clinton mean when she complains about immigration enforcement that “breaks up families”? Vaughan said that if illegal immigrants get deported, they are perfectly free to take their U.S.-born children with them. “Any parent would feel the same way,” Vaughan said. “The responsible response would be to bring the child with you … It’s not the immigration system that’s breaking up the family. That’s the choice the immigrant made by coming here illegally.”

5.) What is the proper level of legal immigration? Currently, about 1 million people legally move to the United States every year. Polls have suggested that Americans favor current or lower levels. Clinton has not specified a figure but favors a number of measures that seem likely to increase the number of foreign-born U.S. residents.

6.) Where do the candidates stand on so-called “sanctuary cities”? Trump’s view is crystal clear — he’s against them. Clinton’s view is more murky. Last year, she criticized San Francisco for ignoring a federal request to hold an illegal immigrant accused of killing Kate Steinle. Yet her campaign maintains that she believes sanctuary city policies improve public safety by encouraging illegal immigrants to report crimes without fear of deportation. That puts her at odds with President Obama’s Department of Homeland Security, which states on its website that uncooperative local law enforcement agencies are a “significant factor” impacting deportation.

A question on sanctuary city policies would have been particularly illuminating — given that Clinton campaign manager Robby Mook could not provide clarity when asked about the candidate’s position over the weekend.

But Holt did not ask that or any other immigration question. Maybe next debate.