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.