If you want to know the reason behind Ben Carson’s tumble in the polls, you need only look at what propelled him there in the first place: race.

Earlier this fall, I argued that many Republicans rallied around the famed neurosurgeon because of their eagerness to show that they have their own brilliant black man to support. Their answer to President Obama. The NBC News-Wall Street Journal poll released late October had Carson (29 percent) as the first choice of registered voters who said they would vote in the Republican primary. The Post-ABC News poll last month put Carson in a commanding second place at 22 percent.

But if your popularity is largely built on race, if you’re going to be “their black guy” in the GOP, you have no margin for error. You have to be damned near perfect in the eyes of the overwhelmingly white party to hold its support. And Carson hasn’t been since that last poll. Carson goes into Tuesday night’s Las Vegas debate a fallen star. The latest NBC News-Wall Street Journal poll released on Monday shows an 18-point plummet for Carson. His support now sits at 11 percent.

Republican presidential candidates face off Dec. 15 for the last time this year. Here's what you should look out for at the prime-time CNN debate. (Sarah Parnass/The Washington Post)

You can pinpoint the beginning of Carson’s slide at the Nov. 10 debate in Milwaukee. There, he gave an incomprehensible answer about Obama’s moves in the Middle East. The one where Carson erroneously said the Chinese were in Syria. That was just three days before the Paris terrorist attacks. The fear unleashed by the slaughter of innocents on Nov. 13 in Paris and on Dec. 2 in San Bernardino, Calif., pushed the presidential campaign into a more serious (and rhetorically scary) phase. Terrorism is now the No. 1 concern of Americans in the surveys from NBC News-Wall Street Journal and Gallup polls.

Don’t forget that the murderous rampage committed by Syed Rizwan Farook and Tashfeen Malik occurred the day before Carson appeared before the Republican Jewish Coalition gathering in Washington. As a result, his Dec. 3 speech, with his continual mispronunciation of Hamas in a performance worthy of a grade-school history class, was more of a disaster than it would have been in less frightening times. And informing the amateur-hour atmosphere around Carson was the devastating New York Times story of Nov. 17 about how he “is struggling to grasp foreign policy.”

Not only has Carson fallen in the NBC News-Wall Street Journal poll released Monday, he has collapsed in the Post-ABC News poll released Tuesday morning. His standing has been cut in half to 11 percent.

Harvard professor Leah Wright Rigueur agreed with my race-based explanation for Carson’s diminished support. “You have to be extraordinary,” she told me via email, “and not just in one area (like brain surgery and neuroscience) but everywhere.” As the author of “The Loneliness of the Black Republican,” Rigueur has been my go-to person to explain the appeal of Carson to the GOP primary voter.  

“Republican voters like the ‘idea’ of Carson, but they’re demonstrating that they’re not willing to support the reality of his candidacy,” Rigueur said. “In fact, they are more than willing to shift their votes to [Donald] Trump, who a month ago, launched a pretty nasty racialized attack on Carson.”

“Race is integral to Carson’s fall, but not in an explicit sense,” Rigueur said, noting that the former leading GOP candidate “still presents a conservative racial counter-balance to Republican voters” and remains beloved for his gentle manner and strong evangelical ties. “That being said, those same voters have said from the very beginning that they weren’t resolute in their support for Carson. They said they could still change their minds; in other words, they weren’t loyal. And we are seeing that play out now; their breaking point was his weakness on foreign policy.”

The lack of loyalty to Carson “is where race plays a central role,” Rigueur said. “People touted their devotion to Carson, but have quickly shifted their loyalty to Trump (or [Ted] Cruz, who has really benefitted the most from Carson’s fall from the limelight).” She also pointed out that Cruz’s highlighting the case of Sabina Loving and Trump’s declaration that his goal is to win “100 percent” of the African American vote and his very public meeting with black ministers “[have] given those candidates the kind of superficial ‘cover’ that Carson had previously enjoyed.”

“In other words,” Rigueur said, “Carson’s former supporters can flock to other candidates and just as easily use their platforms as a buffer against charges of racism.”

“I’m sure if you polled Carson defectors,” she continued,  “they’d argue that race was never a factor.” Of course not. But when their perfect antidote to Obama proved not to be, they had no problem drifting away from him. Carson has made as many incendiary statements as Trump, who has made as many foreign policy mistakes as Carson. And yet Trump’s grip on first place gets stronger by the day. There is only one explanation for it. 

Follow Jonathan on Twitter: @Capehartj