DES MOINES, IOWA — The battle for the White House begins in earnest Monday in the humble Midwestern state of Iowa.

For the last 40 years, Democratic presidential wannabes have spent millions of dollars and a disproportionate amount of time lobbying voters in the Hawkeye State for their support in the Iowa caucuses.

So why is this race on a freezing winter night so important — and why are candidates so desperate to win it?

What are the Iowa caucuses?

The Democrats’ Iowa caucuses are held in January or February of an election year, and mark the start of the primary election race.

The contest culminates with one presidential candidate becoming the party’s official nominee at the Democratic National Convention.

Unlike other states, which have primaries where voters head to the polling booth to cast a ballot, Iowa has used caucuses to select candidates for office since the 1800s, according to the Des Moines Register.

Candidates are fighting for delegates, who will go on to represent them at the Democratic National Convention in June.

This year’s Democratic nominee must win at least 1,991 delegates at the convention.

Iowa has only 41 delegates, but the caucus is crucial because it gives the winner momentum at the start of the season.

How do the Iowa caucuses work?

Republicans in Iowa vote in their caucuses as they would any other standard primary election — by casting a ballot.

But the Democratic caucuses are a byzantine system where voters literally stand in groups to show support for their candidate.

Beginning at 8 p.m. ET Monday, voters will gather at one of 1,678 precincts — held in schools, churches and community halls — where they will cast a vote by standing in a group for their chosen nominee.

Candidates must receive 15 percent of the vote at any precinct to be considered viable.

After a head count, voters who backed a losing candidate can join another group and support a different candidate — sparking a fierce round of courtship from rivals — or call it a day and go home.

This process is why presidential wannabes also lobby to be an Iowan’s second choice.

When the final votes are counted, candidates are awarded a certain number of state delegates ahead of the 2020 Democratic National Convention in June.

Why are the Iowa caucuses so important?

While Iowa only has 41 Democratic delegates — California, by contrast, has 416 — every candidate is desperate to score them.

The victor receives a huge boost in media attention and fundraising before the New Hampshire primary on Feb. 11. It also starts winnowing the field of candidates, which is particularly bloated this election.

If a candidate performs better than expected, it can change their fortunes. This is known as the “expectations game.”

Then-Illinois Sen. Barack Obama defied the odds when he beat Hillary Clinton in Iowa in 2008 and eventually went on to win the nomination.

He credits his momentum in Iowa for propelling him to the White House, the Register reported.

This year’s underdogs, such as Sen. Amy Klobuchar and Pete Buttigieg, will undoubtedly be hoping for the same surprise result in 2020.

What are the latest polls?

After months in second or third place, Vermont Sen. Bernie Sanders has pulled away from the pack and is currently considered the favorite to win Iowa.

The latest New York Times/Siena College poll has the septuagenarian senator 7 points ahead of former Vice President Joe Biden, while an Emerson poll has him 9 points ahead.

RealClearPolitics, which provides a polling average, had Sanders in the lead Sunday with 24.6 percent, followed by Biden at 20.2 percent, Buttigieg at 15.4 percent and Sen. Elizabeth Warren in fourth with 15 percent.

How are the 2020 Iowa caucuses different from 2016?

For the first time, the Iowa Democratic Party will release data from the first round of votes — showing whom people aligned with before their favorite was deemed nonviable or they were persuaded to back another candidate.

The party will also show how many delegates each candidate won. That means that if Biden loses Iowa but garners a similar number of delegates to Sanders, he can trumpet that as a quasi-win heading into New Hampshire.

Why is Iowa first in the US?

The Democratic Iowa caucuses have been the first nominating contest in the US since 1972 after a wild brawl broke out at the Democratic Nominating Convention in Chicago in 1968.

After a tumultuous year that included the ongoing Vietnam War and the assassinations of the Rev. Martin Luther King Jr. and presidential candidate Robert Kennedy, voters were upset that Democratic leaders had elected a pro-war candidate behind closed doors.

Party leaders re-evaluated and decided to open up the caucus process, with Iowa’s primary happening to come first.

The state — which is disproportionately white and rural — has been criticized in recent years for not being representative of the rest of the nation demographically.