Democratic and Republican presidential candidates scrambled across Iowa on Sunday to close the deal with the first voters to have a say in the 2016 race for the White House, urging their supporters to take part in Monday's caucuses in which outsiders Bernie Sanders and Donald Trump are pinning their hopes on a large turnout.

The caucuses will provide a big test of whether the large enthusiastic crowds turning out at rallies for Trump and Sanders will turn into actual votes when Iowans gather on a wintry night for meetings at schools, libraries and even private homes in the first in a series of state-by-state nominating contests.

However, the direction of American politics may all come down to a snowstorm, which is due to sweep in from the west and dump six inches of snow on the state by Tuesday.

Trump and Sanders are dependent on support from younger voters and anti-establishment types who tend to turn up less reliably - especially in poor weather - than the party faithful backing Clinton and the more traditional Republicans.

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The caucuses will provide a big test of whether the large enthusiastic crowds turning out at rallies for Trump and Sanders will turn into actual votes. Trump is seen speaking during a campaign rally in Clinton, Iowa, on Saturday

Sanders, a self-described democratic socialist, is depending on enthusiastic young voters to turn up in greater numbers at the Democratic caucuses. He is seen Saturday in Cedar Rapids, Iowa

Iowa offers only a small contingent of the delegates who will determine the nominees at each party's national nominating convention in July. But those candidates exceeding expectations will gain a burst of momentum heading into New Hampshire with its Feb. 9 primary and other early voting states. The caucus results should also help winnow the crowded Republican field of nearly a dozen candidates.

In the last major preference poll before the caucuses, Trump had the support of 28 percent of likely caucus-goers, with Texas Sen. Ted Cruz at 23 percent and Florida Sen. Marco Rubio at 15 percent. The Iowa Poll, published by The Des Moines Register and Bloomberg, also found Hillary Clinton with 45 percent support to Vermont Sen. Sanders' 42 percent. The poll of 602 likely Republican caucus-goers and 602 likely Democratic caucus-goers was taken Tuesday to Friday and has a margin of error of plus or minus 4 percentage points.

Cruz, who describes himself as a 'consistent conservative,' is relying on a strong get-out-the-vote operation, particularly among the key bloc of evangelical voters who reliably turn out for Republican caucuses. Trump is hoping his star power will encourage a large turnout of first-time caucus-goers.

A similar dynamic is playing out on the Democratic side where Sanders, a self-described democratic socialist, is depending on enthusiastic young voters to turn up in greater numbers at the Democratic caucuses. Clinton has more support among older voters who regularly show up for the caucuses.

'People are really enthusiastic, and if people come out to vote, I think you're going to look at one of the biggest political upsets in the modern history of our country,' Sanders, a self-described democratic socialist, told CNN's State of the Union.

Clinton, the former Secretary of State, said she had been subjected to 'years of scrutiny, and I'm still standing.' On ABC's This Week, she said, 'I feel vetted. I feel ready. I feel strong, and I think I'm the best person to be the nominee and to defeat whoever they nominate in November.'

On the Republican side, Trump said 'I don't have to win' in Iowa, before adding that he believes he has 'a good chance' of victory.

Hillary Clinton has more support among older voters who regularly show up for the caucuses. She is seen on Saturday in Cedar Rapids

Both former President Bill Clinton and the couple's daughter Chelsea Clinton campaigned on behalf of Hillary Clinton on Saturday

The billionaire reality TV star said he was confident of taking New Hampshire and many other contests down the road. 'One of the reasons that I'll win and, I think, none of the other guys will win is because I'm going to get states that they'll never get,' he told CBS' Face the Nation, citing Pennsylvania, Ohio, Michigan, Florida, along with strong hopes for New York and Virginia.

Trump predicted that 'many' senators 'soon' would endorse him rather than Cruz. Trump didn't name any such senators, and none immediately emerged.

Cruz is conceding nothing, saying he's bringing 'the old Reagan coalition' together.

'We're seeing conservatives and evangelicals and libertarians and Reagan Democrats. And if conservatives come out, we're going to win tomorrow,' the Texas senator told Fox News Sunday.

Cruz is conceding nothing, saying he's bringing 'the old Reagan coalition' together. He is seen in Ida Grove on Saturday

Marco Rubio told CNN that as voters learn more about Cruz's record, they will understand that 'he's always looking to take whatever position it takes to win votes or raise money'

Cruz directed much of his final advertising against Marco Rubio as the senators' feud grew even more bitter in the final day before the caucuses.

Cruz took to the airwaves to challenge the conservative credentials of Rubio. One ad said of Rubio: 'Tax hikes. Amnesty. The Republican Obama.'

Rubio shot back, telling CNN that as voters learn more about Cruz's record, they will understand that 'he's always looking to take whatever position it takes to win votes or raise money.'

Rubio said Republicans won't beat Clinton 'with someone that will say or do anything to get elected.'