The question landed with a thud, jarring in its bald bigotry: As President, would you order spying on mosques? 

The second-tier Republican presidential candidates scrambled to answer. But reaction on social media was even faster. Viewers slammed the question as biased, even unconstitutional in its suggestion that houses of worship should be invaded to preemptively monitor potential acts of violence.

The incident illuminates an under-discussed aspect of presidential debates: The power of debate moderators to set the terms of discussion.  After all, the question could have been “Would you order spying on a church?” 

That question would arguably have been just as relevant. Reports from the Department of Homeland Security and others show that numerous acts of domestic terrorism have been conducted by self-identified Christians. 

But that wasn’t the question that debate moderator Martha MacCallum chose to ask. And in her choice, she set an invisible boundary that shaped candidates’ answers. 

MacCallum wasn’t doing anything unusual. For generations, it has been a familiar pattern. Journalists ask the questions, and the candidates answer. Today, there is a wild card in the mix: technological tools such as Twitter that allow a rowdy, freewheeling public to annotate and critique the terms of the debate.

This is something new. Traditionally, a core function of journalism has been to define the terms of acceptable discussion -- although it is rarely described that way. This phenomenon was first identified by political scientist Daniel C. Hallin. He described journalism as covering stories from one of three different perspectives, or spheres. 

Imagine a doughnut: The inner circle, or hole, is the “sphere of consensus.” Topics in this area are discussed with an assumption of widespread agreement. Think Mom and apple pie, or similarly safe opinions.

The middle ring of the doughnut is the “sphere of legitimate controversy.” Here is the lively arena from which presidential debate questions are drawn. Journalists look to familiar themes such as government spending and foreign policy to ask questions such as: Should the US intervene in Syria? How should we address immigration?

Finally, on the outer edge is the “sphere of deviance.” Here are the topics too outrageous to grant legitimacy.  Debate moderators might be aware of these topics, but would consider them too outlandish to voice. 

Issues can move from one sphere to the other. The topic of birthright citizenship, which has been considered settled law in the US for nearly 150 years, was recently presented as an open question to Republican presidential candidate and current New Jersey Governor Chris Christie. 

Importantly, these shifts often happen without explicit acknowledgement that they are occurring. An issue previously considered fair game for debate may slip quietly into the sphere of deviance, where journalists fear that simply asking the question makes them appear to take sides.

For example, the advocacy group ScienceDebate.org found that just 6 of 300 questions submitted by journalists during the 2008 presidential election were about climate change. There are many potential explanations for the absence of such questions, of course, but one is that journalists were afraid of being labeled partisan simply for raising the topic. 

So what does all this mean for the public? Now that the 2016 presidential debate season is underway, it’s worth not just talking about what the candidates said, but what questions they were asked, and by whom.  Spelling out the power that media commentators have to frame the debate is important.  

In addition, social media provides unprecedented opportunities for real-time audience pushback during debates. Viewers today can instantly call out the invisible choices that debate moderators are making, amplify a question that was asked, or suggest an alternative. 

Explicitly identifying journalists’ role in shaping the discussion is unusual, but some are already embracing the idea.  Take this tart observation from a college student watching the recent presidential debate: “Interesting when the moderator is the biggest extremist on the stage.”