Hillary Clinton is tied with Barrack Obama in Iowa according to opinion polls [Reuters]

The state of Iowa has been the scene of furious political campaigning as hopefuls made one last bid for support before the first electoral test in this year's long race to be the next president of the United States.

Shuttling between towns, the 16 candidates from the two parties made last-ditch attempts to win the votes of political activists who will cast their ballots on Thursday.

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A new opinion poll released on Wednesday showed that both the Democrat and Republican caucus races in the state were virtually deadlocked.

After an already long and expensive campaign Iowa is the first test of the state-by-state battle to choose presidential candidates for the election in November.

Barack Obama, the Democratic senator for Illinois, told a rally in Davenport: "I promise you this - we will not just win the caucus, we will win the primary, we will win the general election and we will change this nation and change the world."

Three-way race

Obama is tied at 28 per cent with Hillary Clinton, a New York senator and early Democrat frontrunner, according to the poll conducted by Zogby.

John Edwards, the former North Carolina senator and 2004 vice-presidential candidate, was close behind with 26 per cent.

The poll predicted the Republican race would be equally as close with Mitt Romney, the former governor of Massachusetts, appearing to have closed the gap on the surprise frontrunner in the state, Mike Huckabee.

Huckabee, the former governor of Arkansas, led Romney by two points, well within the poll's margin of error.

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The Democratic caucus begins at 6:30pm (00:30 GMT), with Republicans starting 30 minutes later. Results could begin to appear within an hour or two.

Clinton, Obama and Edwards all bought time on Iowa television to offer closing messages to voters on Wednesday night.

"If you stand with me for one night, I will stand up for you every day as your president," Clinton said in her two-minute message to air on Iowa stations.

Edwards, in the midst of a 36-hour tour around the state, campaigned through the night.

He has been rising in popularity according to polls and drawn criticism from both Clinton and Obama in recent days.

"Most of the other major candidates have been spending a lot of time talking about me," Edwards said on ABC's Good Morning America.

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"I think the reason for it is pretty straightforward. They can see this movement and explosion that's happening in my campaign right here and I think they're trying to blunt it," he said.

On the Republican side, Huckabee has been criticised recently after a news conference on Monday where he announced he would not air an advertisement critical of Romney, but then showed it to reporters.

The incident followed several days of attacks from Romney, who criticised Huckabee's record on immigration, taxes and crime while governor of Arkansas.

"I might have done it differently, but here's the real issue, the issue is we pulled ours. We didn't run a negative campaign. Mitt Romney did," Huckabee said.

The caucus system requires voters to show up at about 1,800 precinct meetings throughout Iowa's 99 counties, to personally vouch for favoured candidates.

Democrats hold open meetings. Republicans use a straw poll.

A new opinion poll also previewed the next key vote in New Hampshire on January 8, and showed John McCain, the Arizona senator and Republican candidate, making a strong comeback.

McCain has largely sacrificed campaigning in Iowa to concentrate on New Hampshire and was tied in a CNN/WMUR poll in the northeastern state with Romney, who had led for months there, on 29 per cent.

Huckabee's lead in Iowa opinion polls has been cut in recent days [Reuters]

Source: Agencies