WASHINGTON (AFP) - Conservative US Senator Ted Cruz has surged to lead the field of Republican presidential contenders in the politically-crucial state of Iowa, a new poll showed on Monday (Dec 7).

The poll - the first in which Mr Cruz has led the field - shows the Texas senator with 24 per cent support from voters who intend to take part in the Feb 1, 2016 Iowa caucuses, the first real measure of voter support in the 2016 presidential campaign.

As recently as October, Mr Cruz, 44, had just 10 per cent support in the Monmouth poll.

His fortunes have changed, however, after a critical endorsement from a popular Republican lawmaker in this heartland state, and as voters cool on retired neurosurgeon Ben Carson, an earlier favourite.

"This marks the first time Ted Cruz has held a lead in any of the crucial early states," said Mr Patrick Murray, director of the independent Monmouth University Polling Institute in West Long Branch, New Jersey, highlighting what he called a "Cruz surge in Iowa".

As Mr Carson's stock has fallen, Mr Cruz has been able to corral most of those voters, Mr Murray said.

Billionaire real estate mogul Donald Trump was the second choice of Iowa Republicans with 19 per cent support, followed by Mr Marco Rubio (17 per cent); Mr Carson, (13 per cent); Mr Jeb Bush, (six per cent); Mr Rand Paul (four per cent) and Ms Carly Fiorina and Mr John Kasich each with three per cent.

Two months ago, Mr Carson topped the Monmouth survey with 19 per cent support, but his poll numbers have been in free-fall in recent weeks following a number of gaffes in the area of foreign policy and security.

His slip-ups have occurred at precisely the moment when Americans are paying more attention to candidates' foreign policy acumen because of perceived security threats abroad and at home.

Mr Rubio, the fresh-faced senator from Florida, is seen by many as the favourite of the party establishment given the lackluster showing on the campaign trail so far of his state's former governor, Mr Jeb Bush.

Like Mr Cruz, he appears to have improved his standing with Iowa voters, increasing his support by seven percentage points since the last poll, up from 10 per cent in October.

The telephone survey of 425 likely participants in the Iowa caucuses has a margin of error of plus or minus 4.8 per cent.