It might seem as if Donald Trump has been in office for eight years already, but we assure you, it's not even been four yet.

But the race to try and boot him out of the White House is already underway - with 11 Democratic candidates - and a couple of Republicans - lining up to challenge him for the Presidency.

Tonight, members of the Democratic and Republican parties in Iowa will meet up to cast their first votes to determine which candidates go forward to the general election in November.

And while the Republican nomination is all-but sewn up for the incumbent - it's still going to be a wild ride for the Democrats.

Here's who's standing, how the caucuses work and everything else you need to know before tonight's Iowa Caucus.

What time is the Iowa Caucus?

Voting opens in the Iowa caucuses at 7pm Central Time - that's around 1am in the UK.

We're expecting results to come in any time after 9pm Central Time - or about 3am our time, but it could be anywhere up to 5.30am before we have a definitive result for both parties.

How does the nomination process work?

Primaries and caucuses

Before each general election, the major parties pick a candidate to run for President. This is done through an excruciatingly long process of primary elections and caucuses, which can last twice as long as the election campaign itself.

Party members in each of the 50 states plus the District of Columbia, American Samoa, Puerto Rico, the American Virgin Islands and Guam vote for their favoured candidate in either a primary election - a regular 'secret ballot' style election held across the state, or, as is the case in Iowa, a caucus, which is much more interesting.

In a caucus, party members in each district go to a public place - like a school hall or community centre to vote.

In perhaps the purest display of democracy known to man, neighbours and local activists are free to lobby other party members into supporting a particular candidate.

In the Democratic Caucus, members publicly show their support for their favoured candidate either by raising their hand or standing in a different corner of the room.

A candidate needs 15% of the vote to be considered "viable". With each round of voting, the "unviable" candidates are excluded from the caucus. Supporters of the viable candidates then jostle to lure fans of unviable candidates to their corner.

There are still 11 candidates in the race to be the Democratic nominee - and while none of the front-runners are going to be in a huge amount of trouble if they don't make a breakthrough tonight.

But winning in either Iowa or New Hampshire is usually an indication of who the eventual nominee will be.

Republicans hold a more traditional ballot, but members still have to be in the room at the same time. As the incumbent, Donald Trump has all-but got his nomination sewn up.

Delegates

Each state has a certain number of delegates at the party's National Convention, decided by party based on the population of the state. These delegates are "awarded" to presidential candidates based either on a "winner take all" principle, or proportionally based on vote share.

Superdelegates

Just to make it even more confusing, there are a certain number of delegates from each state who can pledge to whichever candidate they like. They tend to be office holders, ex-presidents, members of congress, committee members and state chairs.

The Conventions

The end of primary season, and the beginning of the election proper is signalled by each party's National Convention.

These are similar to party conferences, but on a much grander scale. The multi-day events are filled with speeches from party grandees, congressmen and presidential candidates.

The Conventions build up to an often dramatic floor vote, which decides which candidate the party will nominate for the presidential race.

Who is running in 2020?

Poll numbers (in brackets) are from the latest RealClearPolitics polling average.

Democrats

Joe Biden (27.2%)

Vice President to Barack Obama for eight years, Joe Biden was born in Scranton, Pennsylvania. Yes, the home of Dunder Mifflin.

Despite relentless attacks by Donald Trump - which essentially landed the President in impeachment - he's a popular figure in the party, and among the frontrunners in the primary.

He's seen as an establishment figure, representing the moderates in the party.

Bernie Sanders (23.5%)

Bernie Sanders is the US Senator from Vermont, and if you want the easy comparison, he's America's answer to Jeremy Corbyn.

He's left-wing, independently minded, anti-war and, let's face it, an older gentleman.

Born and raised in Brooklyn, 78-year-old Bernie Sanders has been a congressman since 1991. He served first in the House, then in the Senate as an independent, before joining the Democratic Party in 2015 in order to run for the Presidency.

He's a socialist - favouring higher taxes, free education and universal healthcare.

Elizabeth Warren (15%)

The Senior Senator from Massachusetts been a strong voice of opposition during Donald Trump's time in office.

And she's been doing well in the TV debates - despite becoming a target for rivals in the early months of the campaign.

Her big proposals have been free public college tuition and eliminating student loan debt, as well as a $20.5 trillion universal healthcare proposal.

Michael Bloomberg(8%)

Billionaire founder and CEO of massive financial news service Bloomberg.

Served three terms as Mayor of New York City - two as a Republican, one as an independent.

But before running for mayor he'd been a lifelong Democrat, and it's to that party he's returned for his 2020 Presidential run.

A lot of people in the Democratic party are asking whether the answer to one New York billionaire in the White House is another, slightly richer New York Billionaire.

Pete Buttigieg (6.7%)

Known as "Mayor Pete", Pete Buttigieg is the Mayor of South Bend Indiana.

He was supposed to be a long-shot, but did exceptionally well in the early campaign TV debates - and now some polls are putting him in position to win in Iowa.

He's big on electoral reform, health insurance reform and tougher gun laws. He's progressive, but anchored in the centre-left.

Andrew Yang (4.7%)

A businessman, lawyer and author, Andrew Yang announced his campaign with the slogan "Make America Think Harder".

His key policy is Universal Basic Income - paying every citizen money every month to insulate them from a future where robots take their job.

Andrew Yang's supporters are known as the Yang Gang.

Amy Klobuchar(4.3%)

A Minnesota senator and lawyer.

She's known as a pragmatist, and is seen as a "rising star" of the Party.

And she gained her most significant exposure during her questioning of Supreme Court nominee Brett Kavanaugh in 2018.

Having explained to Kavanaugh that she had lived with an alcoholic father, she asked him if he'd ever blacked out from booze. This did not go down well with the nominee, who angrily asked her if she had. He later apologised.

Tom Steyer (1.8%)

A hedge fund manager, billionaire and philanthropist.

He left high finance in 2012 to focus on politics and fight climate change - which he's now doing through his non-profit NextGen America.

Tulsi Gabbard (1.5%)

Tulsi Gabbard is US Representative for Hawaii's 2nd congressional district - and a Major in the Hawaii Army National Guard.

A practicing Hindu, she is of Asian, Polynesian, and Caucasian descent.

She was one of the first female combat veterans to be elected to congress, having served in Iraq and Kuwait in the 2000s.

But anti-interventionism is a key policy in her campaign - calling for the US to pull out of Syria and Afghanistan and stop being involved in continuous "regime change wars."

This month she launched legal action against Hillary Clinton, over claims the former Secretary of State made in a 2019 interview that the Republicans were grooming a third party candidate - implying it was Gabbard.

She has also described Hillary as the "personification of the rot that has sickened the Democratic Party for so long.” So all going great between those two.

She drew fire from Democrat colleagues for voting "present" on President Trump's impeachment - essentially abstaining.

Michael Bennet(0.5%)

A US Senator from Colorado. Born in New Delhi, India, where his father was an aide to the US Ambassador.

He's worked for an investment firm, been a school superintendent and was elected to the Senate in 2010.

Deval Patrick (0.2%)

The former governor of Massachusetts.

Patrick is an MD at Bain Capital - the firm Mitt Romney co-founded.

But before he was governor, he was a civil rights lawyer, and was Assistant Attorney General under Bill Clinton between 1994 and 1997.

Republicans

As we say, the Republican candidate is almost certainly going to be Donald J Trump.

But there are two people challenging him in the Iowa caucus - former congressman and Twitter personality Joe Walsh and Bill Weld, the former Governor of Massachusetts.

But despite neither candidate polling higher than 3% nationally, the Trump campaign have kicked into gear early, organising campaign events ahead of the caucus - sending Vice President Mike Pence into town. They even got a rally with the President himself on Thursday.