- Clinton and Trump lead by slimmest of margins before Iowa votes
- Race to the White House: Full Coverage
Des Moines, Iowa: Republican front-runner Donald Trump puts his precedent-shattering campaign to the test on Monday 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.
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Trump: US foreign policy is 'not fair'
Republican presidential candidate Donald Trump takes aim at US foreign allies ahead of the Iowa caucus.
On the Democratic side, 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 November 8 election to succeed President Barack Obama.
Late opinion polls showed Trump, a blunt-spoken billionaire businessman who has never before sought public office, with a small lead on Cruz, a conservative US senator from Texas. Clinton had a slight edge on Sanders, a US senator from Vermont.
But there was no certainty on who would turn up at the caucuses, or how successful Trump and Sanders would be at getting participation from supporters, many of who are new to the process and disenchanted with traditional politics.
Adding to the unpredictability in Iowa was a large bloc of undecided or persuadable voters in both parties. People in the state are accustomed to a long courtship from candidates and are traditionally in no rush to make a commitment.
A Des Moines Register/Bloomberg Iowa poll released on Saturday showed three in 10 likely Democratic caucus-goers and 45 per cent of likely Republican caucus-goers were still uncertain and could be persuaded to switch to another candidate.
Iowans will begin choosing candidates at 7 pm on Tuesday, with results expected within a few hours.
The caucuses will be the first time Trump, a real estate mogul and former reality TV star, has put his appeal to the test.
A win could validate an aggressive campaign that has alarmed many in the Republican establishment, dwarfed the campaigns of many seasoned politicians and has been marked by controversies such as his calls for a temporary ban on Muslims entering the United States and for a wall along the Mexican border. It would put him in a strong position for later nominating contests.
A loss for Trump would dent his self-identity as a winner and create immense pressure for a better performance in the next contests - in New Hampshire on February 9 and South Carolina on February 20.
'A lot of love'
Trump said he was confident that Iowans who have attended his rallies in large numbers would turn out for him on a wintry evening on Monday.
"It's been an amazing journey, and there's a lot of love in those rooms so I would think that they would show up to vote, I hope so," he told NBC's Today program.
Explainer: the US presidential primaries
Ever wanted to know how American political parties choose their nominees? Now you can.
The Iowa caucuses give the Midwestern state a political influence far greater than its small size and kick off a primary process that leads to the parties' formal presidential nominations at summer conventions.
The caucuses are a long and sometimes arcane ritual, taking place in 1100 schools, churches and other public locations across the Midwestern state. At least two Republican caucuses will be in private homes and one Democratic caucus will take place at an equestrian centre.
White House hopefuls descend on the state early and often and give people the chance to see them up close, making many Iowans slow to decide who to back.
"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 could quickly begin dropping by the wayside.
Dave Burggren of Asbury, Iowa, a precinct leader for his local Republican caucus, has been to seven or eight candidate events, checking out Trump, Rubio, former business executive Carly Fiorina, and even Sanders, but said he still did not have "any idea" which candidate he would support.
"My philosophy is this: Which piece of candy do I want today?" Burggren said at a Trump event in Dubuque on Saturday.
In Iowa in 2012, nearly half of Republican caucus-goers, 46 per cent, 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 per cent decided in the final three days, the entrance polls showed.
Democrats were only slightly more decisive in 2008, when 11 per cent decided the day of the caucus and 9 per cent decided in the last three days, according to entrance polls.
That sort of uncertainty in Iowa has let to 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.
On the Republican side, opinion polls show Rubio pushing ahead of retired neurosurgeon Ben Carson in a critical fight for third place that could help Rubio consolidate support from mainstream Republicans who are not drawn to Trump or conservative Cruz.
For the Democrats, Clinton needs a win in Iowa to prevent a potential two-state opening losing streak that would raise fresh questions about a candidate who was considered the clear front-runner just two months ago, before a surge of support for Sanders.
Clinton lost Iowa in 2008 and went on to lose a protracted primary battle to Obama.
The former secretary of state told ABC's Good Morning America program that it would be different this time.
"I have a much better organisation, to be just really clear about it. I think we built an organisation using a lot of the lessons learned ... I think I'm a better candidate," Clinton said.