Republican front-runner Donald Trump is putting 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.
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 on Monday (local time), 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 percent of likely Republican caucus-goers were still uncertain and could be persuaded to switch to another candidate.
A win could validate Trump's 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.