Caucuses are simultaneous meetings held across the state at which voters decide which candidate they will back.

Polls suggest a three-way battle between Hillary Clinton, Barack Obama and John Edwards for the Democrats.

Surveys suggest the Republican battle is likely to be won by either Mike Huckabee or Mitt Romney.

On Thursday evening, Iowans will gather at more than 1,780 locations across the state to choose delegates, the start of a selection process that will culminate in the Democratic and Republican national nominating conventions next summer.

Given the tightness of the races, rivals have been chasing the last wavering voters in a bid to take Iowa, and so give their campaigns crucial momentum.

Senator Hillary Clinton is to make her final argument to Iowa voters with a primetime television advertisement in which she says America is at a crossroads and she alone has the experience to take charge from the start.

"After all the town meetings, the pie and coffee, it all comes down to this: Who is ready to be president and ready to start solving the big challenges we face on day one?"

Her main rival, Senator Obama, who is also scheduled to air a television advertisement on Wednesday evening, has promised "something very, very special" at Thursday's caucuses.

John Edwards has a full-page newspaper advert featuring a testimonial from a laid-off industrial worker, who also appears in a television commercial to back him.

The latest Reuters/C-Span/Zogby poll released on Wednesday indicates a deadlock on 28 points between Mrs Clinton and Mr Obama, with Mr Edwards only just behind on 26 points.

Momentum

The race for the Republican spot is also looking too close to call, with former Arkansas governor Mike Huckabee pitted against Mitt Romney, the former governor of Massachusetts.

Mr Huckabee urged his supporters to get out and encourage others to back him.

"Don't go alone. Take people with you," he said. "Do whatever you've got to do to get people to the caucus who are going to vote for me."

A recorded interview with Mr Huckabee is due to be broadcast on Jay Leno's Tonight Show on Wednesday evening.

Mitt Romney has also been scouring Iowa to drum up support.

The Iowa caucuses and the New Hampshire primary on 8 January are regarded as key for building up momentum in the state-by-state process of selecting presidential nominees.

Candidates who do poorly tend to drop out of the race.

Polls from New Hampshire have indicated that the Republican race is between Mr Romney and Senator John McCain, while for the Democrats Mrs Clinton and Mr Obama lead the field.

(BBC)

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