Voters in Iowa begin the process of choosing the next president on Thursday, with Democrats Hillary Clinton, Barack Obama and John Edwards in a frantic scramble for the top spot in the campaign's first nominating contest.
Polls also show a tight race between Mitt Romney and Mike Huckabee for the Republican lead as the most heavily contested Iowa campaign in history draws to a close.
For the winner in Iowa, the prize is a valuable shot of momentum and at least a temporary claim to the front-runner's slot in their party's nomination battle.
The third-place finisher in the heavyweight Democratic showdown, meanwhile, could find themselves severely wounded heading into the next contest in New Hampshire on Tuesday.
Iowa opens the state-by-state battle to choose candidates in the November election to succeed President George W. Bush, and traditionally has served to winnow the field of laggards and elevate some surprise contenders.
The 2008 campaign is the most wide-open presidential race in more than 50 years, with no sitting president or vice president seeking their party's nomination.
A Reuters/C-SPAN/Zogby tracking poll showed Clinton's support in Iowa slipped slightly and Obama's inched up on Wednesday, leaving them tied at 28 per cent among Democrats, with Edwards just two points behind.
Huckabee, the former Arkansas governor, led former Massachusetts Gov. Romney by two points in the Republican race on the eve of the caucuses in Iowa, with Arizona Sen. John McCain and former Tennessee Sen. Fred Thompson tied for a distant third.
The Democratic caucus begins at 6:30 pm CST (7:30 p.m. EST), with Republicans starting 30 minutes later. Results could begin to appear within an hour or two.
Record turnout is expected for the Democrats, surpassing the 124,000 Iowans who participated in 2004. Republicans are also likely to surpass their record of 87,000 caucus participants in 2000.
Sub-freezing temperatures were predicted for caucus night, which requires Iowans to leave their homes and join their neighbors at a community gathering spot to publicly declare their support for a candidate.
In the final hours before the Thursday night caucuses, the candidates focused on driving home their message and launching a mammoth voter turnout effort.
"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," Clinton, a New York senator, said in a paid two-minute message that aired on local newscasts.
Like Clinton, Obama also bought two minutes of air time on Iowa television stations during evening newscasts to appeal for votes. Edwards aired a one-minute message.
"I will carry your voices to the White House and I will fight for you every day I'm there," Obama, an Illinois senator who would be the first black in the White House, said in his message.
Edwards used a laid-off Maytag factory worker from Newton, Iowa, to make the case he was the candidate who could battle corporate interests and keep US manufacturing jobs.
The 2004 vice presidential candidate, who has made his promise to fight special interests the centerpiece of his campaign, ended a 36-hour marathon bus trip with a Wednesday night rally in Des Moines featuring singer John Mellencamp.
Huckabee, however, left frigid Iowa to fly to sunny California to appear on the first new broadcast of NBC's "The Tonight Show with Jay Leno" since the Hollywood writers' strike forced the program into reruns on November 5.
He had to cross picket lines to make his guest appearance on Leno while expressing support for striking writers, whose contract talks with major studios collapsed last month.
Campaign spokeswoman Kirsten Fedewa said Huckabee "would only agree to join Jay, an active member of the Writers Guild, for the taping after he was assured that no replacement writers were being used in the show's production."