US presidential hopefuls have held their final full day of campaigning before the nominating contest begins in earnest with the Iowa caucuses.
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.
The campaigns made offers of lifts, babysitters and snow shovels in an effort to lure people out to vote on Thursday.
Senator Hillary Clinton made her final argument to Iowa voters with a primetime television advertisement in which she said America was at a crossroads and she alone had the experience to take charge from the start.
"I'll work my heart out to bring the country we love the new beginning it needs and I will be ready to start on day one," Mrs Clinton said.
Her main rival, Senator Barack Obama, was also airing a television advertisement on Wednesday evening.
"This country is ready for a leader who will bring us together," he said.
"That's the only way we're going to win this election. And that's how we'll actually fix health care, make college affordable, become energy independent and end this war."
John Edwards had a full-page newspaper advert featuring a testimonial from a laid-off industrial worker, who also appears in a television commercial to back him.
He said he was "running on adrenalin" as he ploughed through a 36-hour, 16-stop tour, promising to tackle corporate greed.
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.
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."
Mr Huckabee flew to Los Angeles to record an interview for Jay Leno's Tonight Show, a move that his principal rival attacked.
"I guess he's more focused on the caucus in L.A. rather than the caucus in Iowa," said Mr Romney, who was 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.
Mr McCain and fellow Republican candidate Rudy Giuliani were already in New Hampshire on Wednesday, though Mr McCain later flew back to Iowa after polls suggested he could take third place there. ( BBC )