In a November poll by Quinnipiac, Carson was virtually tied with Trump at 23 percent to Trump's 24 percent. Now Sen. Marco Rubio (Fla.) has surged to second place, taking 17 percent support against Trump's 27 percent.
Carson's campaign has struggled to prove the candidate's understanding of foreign policy and national security, which has become a central concern for voters in the wake of the terrorist attack in Paris last month.
Carson, who has been buoyed by the support of social conservatives, has been among the most gaffe-prone candidates this cycle. His support among evangelical Christians now appears to be eroding, as well. Just 19 percent of white evangelicals support Carson’s White House bid, down from 32 percent support. Cruz, who has 24 percent support among evangelicals, appears to have siphoned the bulk of those voters — last month he carried just 16 percent support in that category.
Carson could risk losing more support among social conservatives following comments he made Sunday about the Planned Parenthood shooting in Colorado that have angered antiabortion activists. Asked if he believed critics who said antiabortion rhetoric resulted in the shooting, Carson equivocated — and was later blasted by key members of his base.
"There is no question that hateful rhetoric, no matter which side it comes from, right or left, is something that is detrimental to our society. This has been a big problem," Carson said on CBS’ "Face the Nation." "I think both sides should tone down their rhetoric and engage in civil discussion."
He has since faced significant backlash. Conservative radio host Steve Deace, a prominent Iowan among evangelical voters, published a column Tuesday accusing Carson of providing "ammunition for those opposed to life to target pro-lifers with." The article ran under the headline "Ben Carson throws pro-lifers under the bus."
The Quinnipiac survey of 672 Republicans was conducted from Nov. 23 to Nov. 30 and has a margin of error of plus or minus 3.8 percentage points.