All 435 seats in the House of Representatives are up for election.

If the Democrats want to take control of the House by having the most seats, they’re going to have to claim at least 23 from the Republicans.

That might not be difficult. A large number of Republicans are leaving the House this year, so a lot is up in the air.

Skip leaving the House Representatives step down for many reasons. Some are simply retiring from politics or seeking higher positions. Some oppose President Trump, others have supported him. Others may be leaving to avoid being voted out of office. End of leaving the House

The battle for the Senate, also currently controlled by the Republicans, will be worth keeping an eye on. There, 35 out of the 100 seats are being contested.

Skip 35 out of the 100 seats are being contested. Democrats need to claim two seats from the Republicans in the Senate to to take control. But there are only nine Republican Senate seats being contested this year, and most of those are expected to stay Republican. End of 35 out of the 100 seats are being contested.

So what would happen if the Republicans held the Senate, but Democrats claimed the House of Representatives?

In a word: gridlock.

The risk of a government shutdown could increase hugely with the rival parts of government unable to agree with each other.

Skip government shutdown A government shutdown happens when Congress fails to agree on funding for government operations across the whole country. End of government shutdown

So who is going to win?

If you believe opinion polls, they say Democrats are ahead in many places.

And some experts think this year could see a so-called wave election, with the Democrats set to make major seat gains.

All the signs from the past point towards a defeat for President Trump.

Skip All the signs from the past point The party with a president in the White House has lost an average of 32 seats in the House of Representatives and two in the Senate in every mid-term election since the American Civil War in 1861-65. End of All the signs from the past point

President Trump’s low ratings in the polls could affect support for Republican candidates and energise the Democrats.

Skip President Trump’s low ratings Trump is one of the most unpopular presidents of modern times. His approval ratings have generally hovered around 40% during his time in office. End of President Trump’s low ratings

And let’s not forget the so-called “pink wave”. This year, more female candidates are running for election than ever before.

Skip more female candidates In Michigan, an all-female list of candidates is running for Governor, while Democrat Rashida Tlaib, who is running unopposed for a House seat, will become the first Muslim woman ever elected to Congress. End of more female candidates

There are plenty of reasons we're seeing more women running.

Some candidates say it’s to make up for Hillary Clinton’s defeat in 2016, but anger at the president’s policies is the most common factor.

On the other hand, Republican support may hold up because the US economy is doing well.

Skip the US economy is doing well. Wages in the US have hit a nine-year high under President Trump, rising 3.1% in the year up to October. Unemployment has also continued to fall. End of the US economy is doing well.

What does it all mean for President Trump?

Donald Trump’s White House has been hit by one scandal after another.

Skip scandal His team is being investigated over Russian attempts to interfere in the 2016 election, and President Trump has also been implicated in illegal payments made to women who said he'd had affairs with them. End of scandal

Up to now, leading Democrats have avoided openly talking about removing President Trump or forcing him to resign.

But if this were to happen, a majority of members of the House of Representatives would have to vote for impeachment.

Skip vote for impeachment This happened most recently with President Bill Clinton in December 1998 - he was cleared in a trial in the Senate after being accused of perjury and obstruction of justice. End of vote for impeachment

What would happen next? The president would then be put on trial by the Senate on charges of “treason, bribery, or other high crimes and misdemeanours”.

If a two-thirds majority of senators found him guilty, Mr Trump would be removed from office and replaced by Vice-President Mike Pence.

Skip Vice-President Mike Pence. The last vice-president to replace a president was Gerald Ford, who entered the White House following the resignation of Richard Nixon in 1974. Current Vice-President Mike Pence was previously state Governor of Indiana and is known for his strong conservative views. He has close links to Christian organisations and has described himself as "an evangelical Catholic". End of Vice-President Mike Pence.

How big a deal would that be? Well, no president has ever been removed from office using the impeachment process.

A more likely outcome will see President Trump frustrated, with a Democrat-controlled House of Representatives blocking his plans.

And what happens next?

Once these elections are done, thoughts turn to the next presidential election in 2020.

Skip the next presidential election in 2020. The election takes place on 3 November 2020 but campaigning starts in 2019. President Trump is likely to seek another 4 year term, but his Democrat challenger will be decided by a series of primary elections which start in January 2020. End of the next presidential election in 2020.

As well as the elections for Congress on 6 November, 36 out of 50 state governors are up for election.

Skip state governors In the US each state has an elected governor who is the main spokesperson for the state, oversees state laws and can appoint local officials and judges. End of state governors

Of those 36, 26 are Republican.

When presidential campaigns get under way, governors often play a big part supporting their party’s candidates - by seeking donors or mobilising volunteers.

So a crop of new governors in the mid-terms could have a major effect on presidential campaigning in 2019 and 2020.