It should be a vote with only marginal significance. Iowa, after all, sends a tiny number of delegates to the Democratic and Republican national conventions, which select each party's presidential nominee for the election in November.

But in being the first to cast a ballot, the Iowa caucuses on Monday carry an outsized weight in the long U.S. presidential campaign.

And current polls suggest that no candidate has the race for either party locked up.

In the campaign for the Democratic nomination, former secretary of state Hillary Clinton is leading Vermont Senator Bernie Sanders in the Iowa polls by a narrow margin. As of Jan. 26, website FiveThirtyEight calculates that Clinton has the support of 48 per cent of Democratic voters, with Sanders at 43 per cent.

Other aggregators, like RealClearPolitics and Huffington Post Pollster, put the gap between Clinton and Sanders at about one percentage point.

The race is also close for the Republican nod. Businessman and reality TV host Donald Trump leads the race with an average of 31 per cent support according to FiveThirtyEight, followed closely by Texas Senator Ted Cruz with 27 per cent. Florida Senator Marco Rubio rounds out the top three with 12 per cent.

Trailing these candidates is a parade of Republican hopefuls, none of whom are polling in double-digits.

While primaries and caucuses are notoriously difficult to gauge — turnout is often little better than 20 per cent of each party's registered voters — recent polls have also shown a high degree of inconsistency.

For example, they have given Clinton anywhere from 43 to 59 per cent support against Sanders's 30 to 51 per cent. Trump has scored between 19 and 39 per cent while Cruz has managed between 23 and 34 per cent.

Iowa caucuses down to the wire3:55

First but not most

The Iowa caucuses are only the first salvoes in a campaign that will drag on for months, but they will have a big influence on what happens further down the line.

Only voters registered with each party have the opportunity to vote in the Iowa caucuses. Republicans vote in a secret ballot, with delegates for the national convention being awarded in proportion to the votes each candidate receives. Meanwhile, the Democrats hold a public vote that harkens back to an earlier age.

In the Democratic caucuses, voters attend meetings in which they group around their preferred candidates. Minimum thresholds have to be met for each candidate to continue to the next round, when Democrats have the opportunity to try to woo supporters of their opponents onto their sides (literally). Ballots aren't counted — people are.

Iowa sends only about  one per cent of delegates to the national conventions, but carries an important weight in the presidential primaries because of its lead-off position, something it has jealously guarded since the 1970s.

While a win in Iowa means little in the final delegate count, it has shown the ability to turn around campaigns in which a candidate had previously been written off.

Springboard to the White House?

Over the last 10 elections in which the Iowa caucuses were contested, the state has selected each party's eventual nominee a majority of the time. Al Gore, John Kerry, and Barack Obama all kicked off their successful bids for the Democratic nomination in Iowa — the last two being unexpected victories that changed the contours of their respective races.

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Sen. Bernie Sanders appears to have the momentum in Iowa. (Jae C. Hong/Associated Press)

It's been different for the Republicans. After selecting Bob Dole in 1996 and George W. Bush in 2000, Iowa has backed unsuccessful candidates: Mike Huckabee in 2008 and Rick Santorum in 2012.

Nevertheless, winning early on in primary season sends a strong signal.

With only one exception since 1980, the Democratic and Republican nominee has won either Iowa or New Hampshire, or both. After losing Iowa in 2008 and 2012, for instance, John McCain and Mitt Romney won New Hampshire, the second state to vote, only days later.

Polls only part of the story

But just how predictive are these current polls in Iowa? With six days to go before the 2012 vote, Santorum — who would eventually prevail with 25 per cent of the vote — was pegged to have about 10 per cent support. By election day, the polls were still underestimating Santorum's support, as they did Obama's in 2008.

There are a number of other factors muddying the waters for both the Republicans and the Democrats.

While Trump and Cruz are leading in the polls in Iowa and nationwide, they are far from the favourite choices of the party establishment, which has favoured Rubio and Jeb Bush, the former Florida governor. More locally, Rubio recently landed the endorsement of the Des Moines Register, the largest newspaper in the state.

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Senator Ted Cruz has lost the lead in Iowa in the past few weeks, polls say. (Jason Reed/Reuters)

Also, in a caucus with low turnout, victory often goes to the candidate with the best organization. Cruz is considered to be much better positioned than Trump, who has a light organization on the ground in Iowa. 

Social conservatives like Cruz also outperformed their polls in the Republican caucuses in 2008 and 2012.

For the Democrats, the results in Iowa may prove to be deeply unrepresentative. Sanders is tied with Clinton among white Democrats nationwide, but trails by a margin of 2-to-1 among Hispanics and 3-to-1 among African-Americans who support the party.

Iowa is much whiter than the rest of the country. While Sanders might be capable of pulling off an upset in Iowa and win New Hampshire, which neighbours his home state, he will have more difficulty winning in other regions of the country.

Sanders may even have difficulty delivering on his strong polling numbers in Iowa. Clinton is reported to lead Sanders comfortably among voters deemed more likely to cast a ballot. Support for the self-described socialist is also heavily concentrated among Democrats under the age of 34.

But Sanders has momentum. Only two weeks ago, Clinton was enjoying a double-digit lead over Sanders in Iowa. Trump, too, is picking up steam. Cruz was leading in state polls just two weeks ago.

The results for both the Democratic and Republican nominations could be close — or surprisingly lopsided, such is the uncertainty that exists in the U.S. primary elections. And then it will be off to New Hampshire on Feb. 9, another small, unrepresentative state with an out-sized importance in this marathon campaign.