Donald Trump has by now demonstrated that he’s not merely another fad candidate in the Republican presidential race. Dr. Ben Carson, on the other hand, still has a lot to prove.

At the start of the month, Carson overtook Trump in the national polling average and in the key early states of Iowa and South Carolina. He also moved into a close second behind Trump in New Hampshire. And he seemed to have the potential to rise higher, with Republicans giving Carson the highest marks of any candidate — by far — on personal favorability.

But with this rise to the top has come a new level of scrutiny from the press and from his opponents. And there are now clear signs that Carson is not holding up well under the spotlight.

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Three weeks ago, an NBC News/SurveyMonkey online poll showed Carson tied with Trump at 26 percent. But now the poll shows that Carson has fallen 10 points behind Trump and is tied for second place with a surging Ted Cruz. There is evidence of slippage elsewhere, too.

In New Hampshire, Carson pulled within two points of Trump in a WBUR poll at the start of the month. But this week, that same poll shows Carson falling back to 13 percent — 10 points behind Trump and tied with Marco Rubio. A new Fox News poll in the Granite State has Carson at just 9 percent — tied for fourth with Jeb Bush.

The NBC News/SurveyMonkey poll paints a clear picture of what’s happening: At the heart of Carson’s rise has been the support of white evangelical voters, who hold particular sway in Iowa (where they accounted for nearly 60% of all caucus-goers in 2012) and South Carolina (where they made up 64 percent of the 2012 primary electorate). At the start of this month, Carson was comfortably ahead with white evangelicals, netting 33 percent of their support. But in the last three weeks, Carson has lost a quarter of that support. At the same time, Cruz has leaped from 7 percent to 22 percent among white evangelicals — pulling into a virtual tie with Carson and Trump.

The movement is even more striking with Republicans who describe themselves as “very conservative.” With this group, Carson’s support has been sliced in half in the last three weeks, from 30 percent to 15 percent, while Cruz has jumped from 25 percent to 40 percent.

The impetus for this decline isn’t entirely clear. Aspects of Carson’s life story, which has been crucial to his appeal to religious conservatives, have been challenged by several press reports this month. But Carson dismissed these as ideologically motivated attacks from the liberal media, and conservative leaders and press outlets largely stuck by him. Still, even if that didn’t turn Republicans against Carson personally, the episode may have caused some of them to doubt the wisdom of nominating him for president.

Read the full story at MSNBC.com