Weeks of tough scrutiny are beginning to take a toll: Ben Carson appears to be fading in Iowa, and there are signs he may be hitting the wall in other early states.

A CBS/YouGov poll released Sunday showed Carson slipping below 20 percent in Iowa and to third place behind Donald Trump and Ted Cruz — the first time he's had a rating of less than 20 percent in a major poll there since September. Since his late October Iowa high point, according to the RealClearPolitics polling average, Carson has dropped 8 percentage points in Iowa and 5 points in New Hampshire.

In South Carolina, which isn't polled nearly as much as the other two early states, there aren't as many data points. But after leading the GOP field there with 28 percent in an early November Monmouth University poll, Carson now finds himself a distant second. According to the latest CBS/YouGov poll of South Carolina Republicans, Carson has 19 percent to Trump's 35 percent.

All of that suggests a challenging road ahead for the retired pediatric neurosurgeon, said Patrick Murray, the director of polling at Monmouth University, which surveys in the early states.

Conservative voters “really want an outsider candidate, [but] they want somebody who appears presidential … with a clear fire in the belly, and I think that’s where Carson has been letting them down,” Murray said. He went on to add, ”Carson is a natural fit for conservative voters, but he seems to have faded as recent national security issues highlighted some doubts conservative voters were already having about him.” 

In part, the concern is stylistic. Carson is near-universally described by fellow Republicans as “nice,” and his fans love his gentle, low-key approach. But following the terrorist attacks in Paris earlier this month, there is an increased appetite among GOP primary voters for strong statements and tough talk. While Trump, like Carson, lacks foreign policy and political experience, he has seen his numbers rise in recent weeks, coinciding with responses such as his promise to “bomb the sh** out of” ISIL. Carson also has promised to take the fight to the Islamic State, but so far the message, delivered in quieter tones, isn’t attracting attention in the same way.

“For Carson, it gives him a lot of difficulty,” said Andy Smith, the director of the University of New Hampshire Survey Center. “He’s stumbled on foreign policy in general in debates, he did not focus on it as much going into the campaign, and he doesn’t appear, through personality, as tough as, say, a Donald Trump is and has been on these issues.”

Smith and other New Hampshire Republicans expect that foreign policy and national security will remain top of mind for GOP primary voters in the state. That puts more pressure on Carson to catch up quickly and convey an image as a more serious wartime leader — or risk fading out of the top tier for good, especially in a more moderate state like New Hampshire where Carson's socially conservative approach is out of step with many of the kinds of voters who historically vote in the state's primary.

Some there say it’s already too late.

“Even though, when I heard him yesterday on the Sunday shows, it sounded like somebody was prepping him really well … I think a lot of confidence has been lost in the past week or so,” said a high-ranking New Hampshire Republican, granted anonymity in order to speak freely. “I don’t think he will make it up. I think he’s peaked, and now, if they don’t trust him on one thing, what else can’t they trust him on?”

The Carson campaign didn’t respond to requests for comment for this story.

In South Carolina, seasoned Republicans agree that Carson isn’t breaking through in the debate over taking on the Islamic State — and when he does, he doesn’t appear decisive or experienced.

“He comes across as very intelligent, measured, thoughtful, but what seems to be appealing to people is more sheer will,” said Chip Felkel, a longtime South Carolina operative who currently is unaligned. “The environment right now is one where strength, or bigness of personality, if you will, [matters].”

Carson, he said, is “obviously brilliant, successful in his own right as a surgeon, but he doesn’t come across with strength, as strong-willed.”

It isn't simply Carson's style that's contributing to his stall in the polls — it's the lack of command on matters of foreign policy and national security. He failed to inspire confidence with a meandering answer in the last debate on the Middle East, which many saw as incoherent and inaccurate. Then came a stumbling performance on Fox News in which he struggled to name countries that, as president, he would invite to join a coalition to take on ISIL. Finally, his own foreign policy adviser said Carson had difficulty grasping the Middle East.

Those moments stoked concerns about Carson’s preparedness in all three early states. But where it ultimately may matter most is in South Carolina, a state with big military and veteran populations where voters are particularly focused on national defense.

In the current, post-Paris climate, said Katon Dawson, the former chairman of the South Carolina GOP, “I suspect Carson probably has topped out.”

“It’ll take a little while” for those numbers to show up in the polls, he said, but “debates matter. Carson’s performance in the last debate was subpar, and the Paris incident has changed things.”

Glenn McCall, a Republican national committeeman from South Carolina, said that as recently as this weekend, he has heard an uptick in uncertainty about Carson’s readiness to be commander-in-chief.

“I am hearing [questions about] just [Carson’s] lack of experience, and maybe on foreign policy and if he has that skill set to aggressively take the fight to our enemies who do us harm,” said McCall, who is neutral. “Folks definitely feel he’s a nice man, smart, but they definitely want someone who’s a little more assertive in addressing these issues.”

McCall noted that Carson will have the opportunity to do so when he visits South Carolina next week to attend a “commander-in-chief” forum in Spartanburg. On the stump and in television appearances, too, Carson has been ramping up his discussion of foreign policy, attempting to lay out ideas for tackling ISIL and taking a hard line against allowing in Syrian refugees. 

His team is also reported to be weighing an overseas trip ahead of the Iowa Caucuses to further develop his foreign policy credibility, potentially to Asia, Africa or Australia.

"Every second you're not on the campaign trail is dangerous," said South Carolina GOP Chairman Matt Moore. "That's for any candidate, every second matters between now and February ... [but] if you're perceived as being weak in that area and want to strengthen your knowledge and credentials, maybe it's a trade-off worth making."

J. Ann Selzer, a respected Iowa pollster, said there is still time for any current perceptions to turn around.

"Just because a candidate falls in mutiple polls doesn't mean they're done," Selzer said. "It's a single data point. It really depends on what he does about that slide."