Former Arkansas governor Mike Huckabee and Senator Barack Obama of Illinois are the likely winners of Thursday's Iowa nominating caucuses, according to The Des Moines Register's final Iowa Poll.

On the Republican side, Huckabee captured 32 per cent of "likely" Republican caucusgoers, ahead of former Massachusetts governor Mitt Romney at 26 per cent, according to the new poll.

Meanwhile, Democratic presidential contender Obama also garnered 32 per cent of "likely" Democratic caucusgoers in Iowa, leading over Senator Hillary Clinton of New York who received 25 per cent of the votes.

The Register conducted the poll on Thursday to Sunday with a margin of error of +/- 3.5 percentage points, meaning that Obama could in theory be tied with Clinton at 28.5 per cent, while Huckabee could lose to Romney.

Huckabee, a former Baptist preacher, has rocketed to the top of Iowa's GOP presidential contender list based on the support of the state's large population of evangelical caucusgoers. Anywhere from 30 to 50 per cent of Iowa's voters are evangelical Christians, according to The Associated Press.

His sudden rise from an under-funded, no-name candidate to a top Republican contender has taken the heaviest toll on Romney, who is also vying for the votes of conservative Christians. Their political rivalry has caused both to butt heads on policy as well as personal issues.

However, Iowa has a reputation for not tolerating politicians who go negative on their rivals. On Monday, Huckabee called a press conference to announce the launching of his new ad in Iowa, but at the last minute he decided that the ad was too negative. Instead, he used the press conference to announce that the ad will not be aired.

In the ad, Huckabee criticised Romney's record as governor and also questioned his character.

"If a man is dishonest to obtain a job," Huckabee says in the ad, according to The Associated Press, "he'll be dishonest on the job."

The ad was in response to Romney's television ads running in Iowa in which he criticised Huckabee's record as Arkansas governor, including his pardons and commutations of convicted murderers.

"I believe the people of Iowa deserve better, and we are going to try and give them better...," Huckabee said when he announced the ad would not run.

After Thursday's Iowa caucuses, candidates will focus their energy on South Carolina and New Hampshire - two other important states that will hold their primaries in January.