Retired neurosurgeon Dr. Ben Carson has done what seemed unthinkable just a week ago, catching up with Donald Trump in a Republican presidential primary poll.

The survey of Iowa voters by Monmouth University pollsters, released Monday morning, shows Carson and Trump each with 23 per cent of the GOP electorate in the Hawkeye State.

And with former tech CEO Carly Fiorina coming in third with 10 per cent, a majority of Iowa Republicans – 56 per cent in all – say they prefer a White House nominee without any political pedigree at all.

'We are clearly happy with the survey,' Carson national press secretary Deana Bass told DailyMail.com. 'It tracks with the enthusiasm that we see as Dr. Carson gathers huge crowds across the country.'

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CO-LEADER: Dr. Ben Carson is tied for first place in a new Monmouth University presidential preference poll of Iowa Republican voters

STILL CRUISING: Trump hasn't relinquished his lead, but his high-profile willingness to buck the GOP establishment has cleared the way for other mavericks to join him

'This campaign is a marathon, not a sprint,' Bass added. 'So we'll continue working hard and reaching out with a grass roots campaign because Dr. Carson knows what's at stake, our children's future.'

HOW THEY STACK UP: Trump and Carson among GOP groups The Monmouth University Poll reveals that various subsets of Iowa Republicans see the two pack leaders differently: Tea party:Trump 27, Carson 22 Non-tea party: Carson 25, Trump 19 'Very conservative': Carson 24, Trump 23 'Moderate to liberal': Trump 26, Carson 17 Evangelicals: Carson 29, Trump 23 Men: Trump 27, Carson 17 Women: Carson 30, Trump 19 Advertisement

A Trump spokesperson did not immediately respond to a request for a reaction to the volatile poll numbers.

Carson's surge marks the first time in the past five weeks that any Republican primary poll conducted in New Hampshire, Iowa, South Carolina or Nevada shows Trump failing to own the top spot by himself.

The massacre of political establishment figures continues with Texas Sen. Ted Cruz in fourth place with 9 per cent. Cruz is a hard-charging tea party darling whom party leaders consider too bold an outsider.

Cruz has also built a strong alliance with Trump, with whom he shares a strong right-wing outlook on the contentious topic of illegal immigration.

The Donald may still have a hidden upper hand against Carson in Iowa, according to Monmouth's polling data, since he has a lead among Iowa Republicans who say their presidential preference is set in stone.

Fully 30 per cent of those whose choices are 'locked in' prefer Trump, compared to 22 per cent who want Carson as their nominee.

Carson, the famed pediatric neurosurgeon, has Trump beat, 25-16, among those whose decision reflects only a slight preference or a toss-up opinion.

But as most presidential elections since World War II have come down to a referendum on likeability – Richard Nixon's being the exception – Carson may have his own stealth advantage.

The Monmouth University Poll found the African-American doctor with a staggering 81 per cent favorability rating, compared with just 6 per cent unfavorable.

By comparison, Trump's split is 52-33, Fiorina's is 67-8 and Cruz's is 58-21.

THE SWEET LIFE: Former tech CEO Carly FIorina, shown sampling ice cream last week in Iowa, has leapt into third place there

TOPS AMONG OFFICE-HOLDERS: Texas Sen. Ted Cruz outperformed every other senator in the GOP field and all the governors, coming in at 9 per cent among Iowa Republican caucus-goers

'Trump's support is currently more solid than Carson's, but Iowa voters are still considering quite a few candidates before they come to a final decision,' Monmouth University Polling Institute director Patrick Murray said Monday.

The third tier now consists of politicians once thought to be the most formidable bunch – senators and governors with financial backing and decades of preparation for America's highest office.

Wisconsin Gov. Scott Walker, who led in Iowa six weeks ago, is now stuck at 7 per cent. But the news is even worse – a 5 per cent showing – for former Florida governor Jeb Bush.

Ohio Gov. John Kasich and Florida Sen. Marco Rubio are tied at 4 per cent apiece, while Kentucky Sen. Rand Paul has the support of 3 per cent. The only other pols to top a 1 per cent result were former Arkansas Gov. Mike Huckabee and former Pennsylvania Sen. Rick Santorum with 2 per cent each.

Santorum won the Iowa Caucuses in 2012.

'These results mark a significant shake-up in the leaderboard from Monmouth's Iowa poll taken before the first debate,' said Murray.

'Carson and, to a lesser extent, Fiorina have surged, while Walker has faded into the background.'