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Donald J. Trump has slipped into second place, falling behind the retired neurosurgeon Ben Carson, in a Quinnipiac University poll of Republican caucusgoers in Iowa released on Thursday, marking the first time he has not held the lead in several weeks.

Riding support from evangelical voters and women, Mr. Carson received the support of 28 percent, compared to Mr. Trump’s 20 percent, among Republicans likely to take part in the caucuses. The results are a reversal from a Sept. 11 poll by Quinnipiac that showed Mr. Trump at 27 percent and Mr. Carson at 21 percent.

One poll does not necessarily indicate a trend. But evangelical voters, among the most reliable to turn out in the Republican caucuses, have been a difficult target for Mr. Trump, the bombastic, New York-born real-estate developer and reality television star.

Mr. Trump has dominated practically every national and state poll since he joined the race on June 16.

Still, the poll shows that the field remains fairly unsettled. Only Senator Marco Rubio of Florida and Senator Ted Cruz of Texas crack double digits among the rest of the field, with 13 percent and 10 percent.

Mr. Carson has more than double Mr. Trump’s support among women. Some 28 percent of the likely caucusgoers say that they want a candidate who shares their values, and 84 percent of the Republicans in the poll list Mr. Carson as that candidate. He also leads as the candidate who the voters surveyed viewed as the most “honest and trustworthy,” which 23 percent of caucusgoers said was important.

“It’s Ben Carson’s turn in the spotlight,” said Peter A. Brown, the assistant director of the Quinnipiac University Poll. “As they’ve been pondering for six months, many in the political world still are trying to understand Carson’s appeal and how someone who seems to be operating outside the traditional news media/political environment is doing so well among the most conservative G.O.P. voters.”

Mr. Trump leads when voters are asked who would be best in handling the economy and who has the right experience to be the next president. Mr. Carson leads among those who consider social issues to be the most important issue. Mr. Trump and Mr. Rubio are the leaders on who would do best on foreign policy.

The survey of 574 likely Republican caucusgoers was taken from Oct. 14 through Oct. 20, and had a 4.1 percentage point margin of error.