Retired neurosurgeon Ben Carson has emerged as a leading Republican presidential candidate in Iowa and is closing in on front-runner Donald Trump in the state that hosts the first 2016 nomination balloting contest. 

The latest Bloomberg Politics/Des Moines Register Iowa Poll shows billionaire Trump with the support of 23 percent of likely Republican caucus participants, followed by Carson at 18 percent. When first and second choices are combined, Carson is tied with Trump.

Trump finds himself in a vastly better position than when the previous Iowa Poll was taken. He has become a credible presidential candidate to many likely Republican caucus-goers. The real estate mogul is rated favorably by 61 percent and unfavorably by 35 percent, an almost complete reversal since the Iowa Poll in May. He finds his highest ratings among those planning to attend the caucuses for the first time (69 percent) and limited-government Tea Party activists (73 percent). Just 29 percent say they could never vote for him, a number cut in half since May.

Although he isn't generating the headlines enjoyed by Trump, Carson has quietly built a dedicated network of supporters in Iowa. During the past month, he also aired more ads than any other presidential candidate in Iowa. Carson has the highest favorability rating among Republican candidates, with 79 percent of likely GOP caucus-goers seeing him positively.

Those glowing views of Carson, who has a compelling life story and is seeking to become the nation's second black president, could make it hard for Trump or other rivals to attack him as the campaign heats up this fall. Christian conservatives, who represent nearly 40 percent of likely caucus participants in the poll, may be starting to coalesce around the former director of pediatric neurosurgery at Johns Hopkins.

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The poll displays the political benefit, at least for now, of not being part of the Republican establishment. When their totals are combined, Trump and Carson—two men without any elected experience—are backed by more than four in 10 likely caucus participants. Add in former Hewlett-Packard CEO Carly Fiorina, who also has never held elective office, and Texas Senator Ted Cruz, who is running an explicitly anti-establishment campaign, and the total reaches 54 percent of the likely electorate.

"Trump and Carson, one bombastic and the other sometimes soft-spoken, could hardly be more different in their outward presentations," said J. Ann Selzer, president of West Des Moines-based Selzer & Co., which conducted the poll. "Yet they're both finding traction because they don't seem like politicians and there's a strong demand for that right now."

Wisconsin Governor Scott Walker, the previous Iowa front-runner, has been hurt the most by the Trump and Carson summer surges and is now backed by just 8 percent of likely caucus-goers, less than half what he recorded in the last Iowa Poll in late May. Cruz, who will need to cut into Carson's support among social conservatives if he's to advance in Iowa, is tied with Walker at 8 percent.

Jeb Bush, who continues to face major headwinds in Iowa, scored below Walker and Cruz. The former Florida governor is backed by just 6 percent, has one of the highest unfavorable ratings among the 17 Republican candidates tested, and has the support of just 16 percent of those who consider themselves business-oriented establishment Republicans, the group most central to his brand.

Bush's fellow Floridian, Senator Marco Rubio, is also backed by 6 percent. He's closely followed by Fiorina, who is supported by 5 percent after her strong showing in the Aug. 6 debate.

In the 2008 and 2012 Republican caucuses, Christian conservatives broke late in the race and helped determine the outcome in Iowa. While some of their leaders have expressed skepticism about the potential to unify behind one candidate in such a crowded race, there's an opening for that. More than three-quarters of Christian conservatives in the poll say they could be convinced to back someone other than their first or second choice, if they could be assured that another Christian conservative would win.

At the moment, Carson is leading with voters in that bloc at 23 percent, followed by Trump at 16 percent and Cruz and Walker tied at third. If his competitors can successfully raise questions about Trump's credentials as a Christian conservative, they could potentially peel off some of the front-runner's support.

One major unknown for the caucuses is the size of the electorate, which has been around 120,000 on the Republican side for the past two Iowa caucuses.  One of Trump's campaign goals is to get thousands of new people to vote, a move that helped Barack Obama score an upset on the Democratic side in 2008.

First-time caucus-goers are clearly an important part of Trump's Iowa base. Among those who say they'll be attending for the first time, Trump is ahead of Carson, 28 percent to 20 percent.

For now, the poll suggests about a fifth of those attending the Feb. 1 precinct meetings will be doing so for the first time. That's comparable to four years ago, when 24 percent said that on the Republican side in an October 2011 Iowa Poll.

Trump's supporters in Iowa a have a higher level of trust in their candidate than others in the field to make the right decisions, if he makes it to the White House. Among all Republicans likely to attend the caucuses, 41 percent want their candidate to be clear about the specific policies they would address if elected, while 57 percent trust their candidate to figure it out once elected.

For Trump, nearly two-thirds of his supporters trust him to figure out the right decisions once in office. That's in keeping with a claim he made to reporters Aug. 15, shortly after landing by helicopter outside the Iowa State Fair, saying it's mostly the media that cares about policy papers and positions.

Among most of the subgroups measured in the poll, Trump has the advantage, although Carson beats him or comes close with several. Carson has an 11-percentage-point advantage over Trump among seniors and 7-percentage-point-point edge among Christian conservatives.

"I'm sick and tired of the political class," said Lisa Pilch, 54, a middle school physical education teacher leaning toward Carson who lives in Springville, Iowa. "I just like his tone and think he's someone who could pull us together, rather than the polarization we have right now. He has a lot of wisdom, even if he doesn't have political savviness."

While Carson is doing slightly better than Trump among women, the billionaire has the advantage among men, 28 percent to 17 percent.

"He's got a no-nonsense approach," said Patrick Messmore, 32, a construction equipment sales manager who lives near Grundy Center and plans to back Trump. "His history as a businessman is potentially a good change for our country, so that we don't just have another life-time politician taking over as president."

In some ways, Messmore sees Trump as an antidote to Bush. "I'm not OK with another Bush presidency," he said. "We've had two of them now and I don't see that there will be enough of a different approach than his dad or brother had. It’s just not something I'm interested in."

The poll shows Walker and Senator Rand Paul of Kentucky, especially, aren't performing anywhere close to earlier expectations.

Paul, who was backed by just 4 percent, was perceived a year ago to have an advantage in Iowa, given the third-place finish in the 2012 caucuses recorded by his father, former Representative Ron Paul of Texas. In October, his favorable rating outweighed his unfavorable by nearly 3-to-1.

"Whatever advantage he had has eroded," Selzer said. "Now, more Iowa caucus-goers have negative than positive feelings about him."

For Walker, who has been in a slump since his lackluster debate performance, the poll is certain to further reduce expectations around his performance in Iowa, which had grown to the point where anything short of a win would have been viewed as a loss. One upside for him in the poll: Besides Carson, he's the only candidate to exceed 70 percent in favorability.

Iowa Republicans are showing little interest in re-runs. Former Arkansas Governor Mike Huckabee, who won the 2008 Iowa Republican caucuses, is at 4 percent. He's followed at 2 percent by candidates who are governors: Chris Christie of New Jersey, Bobby Jindal of Louisiana, and John Kasich of Ohio.

Former Senator Rick Santorum of Pennsylvania, who narrowly beat eventual nominee Mitt Romney in the 2012 caucuses, is backed by just 1 percent, the same level of support recorded by former Texas Governor Rick Perry, who is also struggling in his second White House bid even amid heavy spending in Iowa on the part of a super political action committee backing him.

Senator Lindsey Graham of South Carolina, former New York Governor George Pataki, and former Virginia Governor Jim Gilmore all recorded support of less than 1 percent.

The survey, taken Aug. 23-26, included 400 likely Republican caucus participants. On the full sample, it has a margin of error of plus or minus 4.9 percentage points.

Besides the nearly the nearly 40 percent of likely Republican caucus-goers who say Christian conservative is the best way to identify them, "business-oriented establishment Republicans" and Tea Party activists are roughly tied as the next largest groups, at 22 percent and 21 percent. Those who feel they are most closely aligned with the "liberty movement," a bloc associated with Paul, represent only about 8 percent.

To offer another assessment of candidate strength—something difficult to divine in such a crowded field—Selzer created an index built on multiple measures in the poll. The index takes into account first and second choices, as well as a question that was asked on whether respondents could ever—or would never—support each candidate they didn’t name as their first or second pick. First choices were given double weight, while “ever support” was given a half weighting.

Using that system, Carson is narrowly ahead of Trump, 75 to 73. Walker comes next at 55, followed by Cruz at 53 and Rubio at 50. The index and never/ever question also show some of the candidates could struggle to expand their support. Nearly half of likely Republican caucus participants, 48 percent, say they could never support Christie. For Paul, it's 43 percent and for Bush it's 39 percent.

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