You might have heard that the 2018 United States midterm elections are only days away.

Key points: The US midterms will determine how easily Donald Trump can push his policy agenda

The US midterms will determine how easily Donald Trump can push his policy agenda The midterms are seen as a referendum on Mr Trump and the direction of the country

The midterms are seen as a referendum on Mr Trump and the direction of the country Issues like healthcare, immigration, and reproductive rights will drive voters to the polls

The midterms on Tuesday, November 6 are said to be the most important in years — some are even calling them the most important in American history.

But what even are midterms? Why are they so important, let alone more important now? And what is going to happen?

Here's what to look for and what to expect as America takes to the polls.

What are the midterms?

The midterms fall half way through a president's sitting term. ( ABC News: Jarrod Fankhauser )

The American midterm elections are held nationwide every four years, halfway through the sitting president's four-year term — hence the name 'the midterms'.

There are all sorts of elected positions on ballots but the main focus is on Congress — America's version of Australia's Parliament — which is made up of the House of Representatives (lower house) and the Senate (upper house).

Representatives in the House serve two-year terms, which means all 435 House seats are up for re-election.

Senators, however, serve staggered six-year terms and 35 Senate seats are up for grabs this fall.

There are also governors' races in 36 states and three American territories this year, and a whole lot of lower-level state and local races that you don't really need to know about — but if you are interested, here's a full list.

Some states are even voting on individual issues, including in Florida where constituents are deciding whether convicted felons who have served their time should regain the right to vote.

What's going to happen?

Mr Trump isn't up for election in the midterms, but the result is important for his presidency. ( Reuters: Carlo Allegri, file )

US President Donald Trump is NOT up for re-election until 2020, but the result will be seen as a marker of his success as President so far.

Unfortunately for Mr Trump, the party of the sitting President almost always loses congressional seats in midterm elections.

This is because midterms are seen as referenda on the current administration, and it's much easier to energise and mobilise voters who lost the presidential election than voters who believe their preferred candidate is doing a great job.

A quick look at the voter turnout at previous US midterms compared to presidential election years show why Mr Trump and Republicans are pushing hard for their supporters to get out and vote.

Voter turnout is normally far lower than in presidential years. ( ABC News: Jarrod Fankhauser )

Five close races to watch Florida's 26th district (House) - Republican-held district that overwhelmingly supported Hillary Clinton in 2016

Republican-held district that overwhelmingly supported Hillary Clinton in 2016 Texas (Senate) - Mr Trump's nemesis-turned-ally Ted Cruz faces stiff opposition from three-time congressman Beto O'Rourke in the traditionally Republican state

Mr Trump's nemesis-turned-ally Ted Cruz faces stiff opposition from three-time congressman Beto O'Rourke in the traditionally Republican state North Dakota (Senate) - Democrat Heidi Heitkamp faces a difficult challenge in a state Mr Trump won by double digits in 2016

Democrat Heidi Heitkamp faces a difficult challenge in a state Mr Trump won by double digits in 2016 Texas' 23rd district (House) - Democrat Gina Ortiz Jones is trying to become first woman to represent district in a seat held by Republicans by just 1.3 per cent in 2016

Democrat Gina Ortiz Jones is trying to become first woman to represent district in a seat held by Republicans by just 1.3 per cent in 2016 Maine's 2nd district - 102 years since an incumbent has been toppled, but strong state Democrat Jared Golden could change history

In the current situation, the Democrats need to net 23 seats to win back control of the House and claim the 218 majority.

The New York Times estimates between 60 and 70 races are closely contested, and that realistically around 30 of those will determine whether the Democrats reclaim the House and by how much.

According to FiveThirtyEight, a leading American politics and data journalism site, Democrats have around an 85 per cent chance of success.

In the 100-seat Senate the current balance of power is very tight, with Republicans holding a narrow 51-49 majority.

To have control of the Senate the Democrats need to gain at least two seats, because Republican Vice-President Mike Pence serves as the tie-breaker when votes are tied.

To regain control from the Republicans, the Democrats need to win two Senate seats and 23 in the House. ( ABC News: Jarrod Fankhauser )

But winning those two seats won't be easy.

Just nine of the Senate seats up for grabs are held by Republicans and, according to FiveThirtyEight, the Republicans have a six-in-seven shot to maintain control of the Senate and are even likely to build on their advantage.

Why do midterms matter?

They matter because whichever party controls Congress control the passage of laws.

Basically, if the Democrats win big, Mr Trump's right-wing agenda would be hampered and he would struggle to enact his desired tax cuts, immigration restrictions, and anti-abortion policies.

But, if the Republicans hold onto both houses, it would embolden the 45th President.

Sorry, this video has expired What's at stake for Trump in the Midterms?

For example, in the first two years of Barack Obama's presidency, he was backed by a Democratic Congress and was able to pass his agenda.

But in the 2010 midterms, Republicans gained control of the Senate and proceeded to obstruct Mr Obama's policy moves everywhere they could.

It's likely the Democrats will use similar tactics should they find themselves in control of Congress.

Outside of Congress, the races for Governor — the equivalent of a State Premier in Australia — also have an underlying importance.

Governors will be in control of drawing electoral district maps following the 2020 US Census.

Sitting governors will be able to manipulate district lines to align with their voter bases — a process known as 'gerrymandering' — which would provide crucial advantages to their party in every state and national election until the 2030 US Census.

This election 36 states will pick a Governor, who are the equivalent of an Australian state premier. ( ABC News: Jarrod Fankhauser )

The position of Governor has also been a traditional proving ground for those with future ambitions for high office or positions within a White House administration.

Seventeen of the 45 US presidents have also served as Governors — think Theodore Roosevelt, Jimmy Carter, Ronald Reagan, Bill Clinton or George W Bush — so it's possible that some of the current crop of contenders may be destined for the Oval Office.

How about the Russia investigation?

If the Democrats win big this time, it is expected that they will move to check Mr Trump wherever they can.

They also have the ability to probe and investigate the Trump administration and his 2016 election campaign.

Democrats would gain control over who to subpoena and where to focus inquiries — a process that has been closely monitored by the current Republican-controlled Congress.

Democrats have already made over 100 formal requests related to the investigations into the Trump administration and the 2016 campaign, which Republicans have mostly deflected.

So with Democrat control of Congress, subpoenas and inquiries could be coming thick and fast for the president and his team.

The midterm elections will be a referendum on the first two years of Donald Trump's presidency. ( Reuters: Kevin Lamarque )

Also, the more Democrats there are in the Senate, the greater the possibility of impeachment.

The threat is not lost on Mr Trump, with Republicans imploring their supporters to vote by threatening that a Democrat-controlled Congress would move to impeach the President.

If they gain control of the House of Representatives — which experts predict they just might — the lower house could easily vote to impeach Mr Trump.

But to remove a President by impeachment, they would need a two-thirds majority in the Senate.

And even if the Democrats win all available Senate seats in the midterms, achieving that majority would not be possible without a stack of Republican votes.

So hypothetically Mr Trump could be impeached, but remain President — just as Democrat Bill Clinton did when he was impeached following the Monica Lewinsky scandal.

What are the key issues being debated?

As is the case in any election there are the main policy issues alongside the personal attacks and underlying tensions.

This year the midterms are being fought on three major policy battlegrounds — jobs, healthcare, and immigration.

The Democrats claim that Mr Trump and his Republicans are trying to gut the country's popular entitlement programs and strip away important healthcare protections, which Republican politicians deny.

The Democrats are also slamming Mr Trump's 'trade wars' for hurting US jobs, while Republicans point to the falling unemployment rate since Mr Obama left office as evidence of their economic prowess.

But dwarfing both issues is immigration, with Mr Trump upping his anti-immigration rhetoric and claiming that the Democrats are behind the caravan of thousands of mainly Honduran migrants heading towards the US-Mexico border.

Sorry, this video has expired The migrant caravan has become a key focus of Mr Trump as it makes its way to the US-Mexico border

While the focus on stopping the caravan will resonate with his Republican base, Democrats will point to border detentions, deportations and the mass separation of migrant children and parents as a hallmark of Mr Trump's immigration policy.

But despite those key policy areas, many will be influenced by Judge Kavanaugh's Senate confirmation hearings, which left a bitter taste in the mouths of Americans on both sides.

Republicans slammed the Democrats for their obstructionist tactics during the hearings, while many Democrats and liberals saw Republican Judge Kavanaugh's appointment in the face of multiple sexual assault accusations as an shining example of the sexism and women's rights issues currently being grappled with in America.

Mr Trump's 2016 triumph despite multiple sexual assault allegations of his own, as well as the growth of the #MeToo movement, has meant he has had to face protests by women worldwide from day one.

Now, on November 6, more women are running for office at every level of American Government than ever before.

Plenty of issues are activating Americans and driving them to the polls, but none more so than how they feel about Mr Trump himself.

The President is still a hugely polarising figure, and whatever the result, it will be seen as a marker of his presidency and his prospects for re-election in 2020.

When will we know results?

Americans have already been out in huge numbers to vote early. ( AP: Jim Mone )

The first results are likely to feed in some time on Wednesday morning AEST as polling stations close.

Analysts will be able to make their predictions, but in a country where sometimes up to half of voters submit their ballots early and by post, it might take days or weeks to know the full picture.

Whatever the result, the fallout will be massive for US politics and the state of the country for the coming years.