Republican front-runner Donald Trump puts his precedent-shattering campaign to the test when Iowa voters begin the nationwide process of choosing a new US president, as polls show a tight battle with Ted Cruz that could hinge on turnout and a large bloc of undecided voters.
Democratic front-runner Hillary Clinton also faces a stiff challenge in Iowa from insurgent Bernie Sanders in the first contest in the state-by-state battle to pick candidates for the 8 November election to succeed President Barack Obama.
Late polls showed Mr Trump, a blunt-spoken billionaire businessman who has never before sought public office, with a small lead on Mr Cruz, a conservative US senator from Texas, while Ms Clinton had a slight edge on Mr Sanders, a US senator from Vermont.
But uncertainty remained about who would turn up at the caucuses, and how successful Mr Trump and Mr Sanders would be at getting their supporters, many new to the process and disenchanted with politics as usual, to participate.
Adding to the unpredictability in Iowa was a large bloc of undecided or persuadable voters in both parties in a state where voters accustomed to a long courtship from candidates are traditionally in no rush to make a commitment.
The Des Moines Register/Bloomberg Iowa poll released on Saturday night showed three in 10 likely Democratic caucus-goers and 45% of likely Republican caucus-goers were still uncertain and could be persuaded to switch to another candidate.
The vast array of choices along with the ability to see the candidates up close over the course of the past year gives many Iowa voters little incentive to rush into a firm choice.
"I'm still checking them out. The field is large and it requires some thought," said Paul Albritton of Carlisle, Iowa, a training coordinator at Iowa State University, as he waited to see US Senator Marco Rubio of Florida last week.
"I'm thinking about who can win in November."
For the winners in Iowa, the prize will be valuable momentum in nominating battles that could stretch for months, while many of the losers on the Republican side quickly could begin dropping by the wayside.
Iowans will attend caucuses at schools, libraries and other public locations beginning at 1am tomorrow Irish time, with results expected within a few hours.
Late decisions
Iowans often make their decisions late. In 2012, nearly half of Republican caucus-goers decided in "the last few days," according to entrance polls of the participants.
In 2008, almost one in five Republicans decided on the day of the caucus, and 13% decided in the final three days, the entrance polls showed.
Democrats were only slightly more decisive in 2008, when 11% decided the day of the caucus and 9% decided in the last three days, according to entrance polls.
"I won't make my final decision until I write the name down on that piece of paper," said Doug Gross, a Republican lawyer in Des Moines who chaired Mitt Romney's 2008 state campaign.
That sort of uncertainty has not led to huge upsets in the more than four-decade history of Iowa's caucuses, but the state has seen late shifts such as Republican conservative Pat Buchanan's charge into second place in 1996 and former Vermont Governor Howard Dean's plunge to third place in the 2004 Democratic race.
"Iowans take their role very seriously, and even in the last week we are learning new things and seeing new positions from the candidates," said Brad Anderson, an Iowa-based Democratic strategist who led Mr Obama's 2012 state campaign.
Iowa will be the first time Mr Trump, a real estate mogul and former reality TV star, puts his appeal to the test.
A win could validate a campaign that has featured his call for a temporary ban on Muslims entering the United States and a wall on the Mexican border, and put him in a strong position for later contests.
A Trump loss would put a dent in his self-identity as a winner and create immense pressure for a better performance in the next contests in New Hampshire on 9 February and South Carolina on 20 February.
Polls also show Mr Rubio pushing ahead of retired neurosurgeon Ben Carson in a critical fight for third place that could help Mr Rubio consolidate support from mainstream Republicans not drawn to Mr Trump or conservative Mr Cruz.
For the Democrats, given Sanders' lead over Ms Clinton in New Hampshire polls, the former secretary of state needs a win in Iowa to prevent a potential two-state opening losing streak that would set off alarm bells at her campaign headquarters and raise fresh questions about a candidate considered a prohibitive front-runner just two months ago.
It was in Iowa in 2008 that Ms Clinton's last front-running presidential campaign went off the rails when she finished third behind Barack Obama and former US Senator John Edwards.
