The Democratic Party's presidential contenders are busy tearing each other apart in a complicated process to decide who will go up against Donald Trump in November for election to President of the United States.

The Democratic primaries, which officially began in Iowa on February 3, are complicated and hugely expensive. They're drawn out too, culminating in the Democratic National Convention in July, a stadium event featuring blizzards of red, white and Democrat blue streamers and balloons.

Democratic Party political elites used to decide the party's nominee with only token input from ordinary voters. That changed following the violent Democratic National Convention of 1968 – a year of all-round civil unrest at the height of the Vietnam War and after Robert Kennedy's assassination – in which Hubert Humphrey was chosen as the party's presidential candidate without winning a primary contest under his name.

Above: TV anchor Dan Rather is jostled to the ground at the chaotic 1968 Democratic National Convention. 

In 1972, the party changed its rules to give voters – rather than political insiders – a dominant say over the process. This trend has continued ever since, and the Republican Party uses a similar process to decide its nominee.

In short, today the successful candidate who emerges in July is decided in an incremental way in the preceding six months through a series of primaries and caucuses. The votes that a candidate receives in these contests allows them to amass "pledged delegates" to support them at the convention. If a candidate secures 50 per cent of these delegates, they become the party's nominee.

At one point last year, almost 30 candidates were running for the nomination but most of them have dropped out. Some simply ran out of money; others decided they had no realistic prospect of emerging the winner. These includes high-profile senators Kamala Harris and Cory Booker and several state governors. Some low-polling candidates – such as businessman Tom Steyer and Hawaii congresswoman Tulsi Gabbard – are still competing even though they seem to have no viable path to victory.

So who is voting for what? Who are the remaining contenders? And what is the difference between a caucus and a primary?

How does the delegate system work?

Delegates are people who are able to vote at the party convention, and there are two groups of them, pledged and super-delegates (more on them later). Pledged delegates represent their state and "pledge" to vote in accordance with their state's wishes (occasionally they defect to another candidate but this is rare).

Each state uses its own rules to determine how its pledged delegates are selected. Some use a "primary" and others use a caucus.

In some states, you must be a registered Democrat to participate in voting; in other states, everyone is allowed to have a say, including Republicans. Some states allow you to mail in a ballot well ahead of polling day; other states do not.

The most populous states send the most delegates to the convention: for example, California sends 415 pledged delegates to the convention while Wyoming sends just 14.

The Democratic Party uses a proportional system to convert the results of the primary/caucus into pledged delegate numbers. This means a candidate could rack up a considerable number of convention delegates without "winning" the majority of votes overall in many, or even any, states.

(The Republican selection process has several differences but most notably, it traditionally uses a first-past-the-post system where a winning candidate receives all delegates from a particular state, regardless of what portion of the votes they attract – although some states favour a proportional approach.)

But, crucially, candidates must win 15 per cent in a contest to claim any delegates for that state.

For example, at the New Hampshire primary in February, Bernie Sanders won 26 per cent of the vote, Pete Buttigieg won 24 per cent and Amy Klobuchar won 20 per cent. This equated to nine pledged delegates each for Sanders and Buttigieg and six for Klobuchar.

Elizabeth Warren and Joe Biden finished beneath the 15 per cent threshold, meaning they did not claim any delegates in this state.

There will be 3979 pledged delegates in total at the 2020 convention in July and only they are allowed to participate in the first round of voting. If a candidate receives a majority of pledged delegates in the first round – that is, 1991 delegates – they are declared the party's nominee.

What happens between now and July?  

 In February, there are four contests: caucuses in Iowa and Nevada, and primaries in New Hampshire and South Carolina.

Because these are the first contests, they get a lot of attention. Candidates who perform poorly in these early states often struggle to maintain the funds required to stay in the race, or cannot see a viable path to the nomination.

But the actual number of pledged delegates up for grabs in February is small: just 155, or 4 per cent, of total pledged delegates.

This is why a candidate such as Michael Bloomberg can skip the early states, opting instead to hit the stage on Super Tuesday, but still hold out hope of being the party nominee.

In the Iowa caucuses, Bernie Sanders won the most popular votes but Pete Buttigieg secured 13 delegates compared to 12 for Sanders and eight for Elizabeth Warren. (The final tally may change because of voting irregularities.)

Sanders then won the most delegates in New Hampshire, narrowly beating Buttigieg. The Nevada caucuses will be held on February 22 and the South Carolina primary on February 29.

The intensity of these contests can't be overstated. In New Hampshire, 5000 people turned out to a rally for Bernie Sanders, who had enlisted rock band The Strokes, Sex and the City actress Cynthia Nixon and high-profile congresswoman Alexandria Ocasio-Cortez to energise his progressive base.

What is Super Tuesday?

On "Super" Tuesday, on March 3, 16 states will hold contests. These include the two biggest states in the country, and those with the most pledged delegates: California and Texas.

In total, 34 per cent of pledged delegates are in play on Super Tuesday. Win big here and you have an excellent chance of becoming the nominee.

What happens up to July?

During March, 13 more states will hold their contests with 27 per cent of delegates up for grabs.

Key states during these dates include Florida, Michigan, Illinois and Arizona.

Some big, delegate-rich states still remain in play in the later stages of the race. New York and Pennsylvania, for example, both hold their primaries in late April.

By the end of May, 95 per cent of pledged delegates will have been awarded and by the end of June all of the delegates will be decided.

Who votes at the national convention in July?

The other, smaller group of delegates are known as super-delegates – elected members of Congress, governors, and former presidents and vice-presidents. These delegates are un-pledged, meaning they can vote however they please.

In a new rule introduced by the Democratic Party this election cycle, they do not participate in the first round of voting. The Democrats changed the rule following complaints from Bernie Sanders' supporters in 2016, who said allowing super-delegates to vote in the first round undermined the will of ordinary party members.

Super-delegates will vote only if a single candidate does not claim a majority in the first round of voting. This is what is known as a "contested" or "brokered" convention. In the second round of voting, all delegates become un-pledged, meaning they can vote as they choose.  

Contested conventions are extremely rare, even after hard-fought nomination battles. The Democratic Party hasn't had one since 1952, before the birth of the modern nominating system.

From the convention, one Democratic presidential candidate will step into the spotlight.

The presidential election will be held on November 3 – but not before more    splashy campaigning from both sides.