This November's US elections are called "midterms" because the come in the middle of the president's four-year term in office.
While the president isn't on the ballot, the midterms are seen as the American voters' verdict on how the president is going and can have a major impact on their ability to legislate and govern.
How many seats are up for grabs?
All 435 members of the United States House of Representatives are elected for just two years to represent a particular district in their state.
All seats are up for grabs at the midterms.
The Senate is 100-strong with two representatives per state.
Senators are typically elected to a six-year term, which means that every two years around one-third of the Senate faces re-election.
Senators who win 'special' elections, such as Georgia Democrat Raphael Warnock, or have been appointed to serve out another senator's term, may have to face voters sooner.
The state of the Senate
Heading into November, President Biden's Democratic party holds fifty Senate seats.
This means they often rely on Vice-President Kamala Harris to break a tie and pass laws through a process called reconciliation.
They sometimes get support from Republicans, but not much.
They are more likely to face a filibuster, which requires a super-majority of 60 votes – making legislating even harder.
The other important thing the Senate does is confirm the president's nominees to become federal and Supreme Court judges.
These are lifetime appointments and can affect the course of the nation for generations.
If the president's party loses their Senate majority, their judicial choices will tend to become more centrist and moderate.
What's happening with the House?
The US House of Representatives isn't quite as tight as the Senate, but almost.
Biden's Democrats hold 220 seats, the Republicans hold 211.
There are four vacancies after one Democrat and one Republican resigned, and two Republicans died during their term in office.
Will history repeat itself?
History suggests the Democrats are in real danger of losing their majorities in both the house and senate.
In the 19 midterm elections since World War II, the president's party has improved its share of the House vote just once – and picked up seats just twice.
In the aftermath of 9/11 and the ousting of the Taliban from Afghanistan, George W Bush enjoyed a 63 per cent approval rating and his Republican party picked up six House seats.
Bill Clinton's approval was three points higher in 1998, and the Democrats netted five seats.
But they were the exception.
Every other modern midterm vote has seen the president's party lose ground, including those with high popularity like Ronald Reagan and George HW Bush
And the big worry for Joe Biden is that with around 42 per cent approval, his popularity is close to where Donald Trump's was four years ago when Republicans lost forty seats.
The average midterm loss for the president's party in the past 50 years is 23 house seats.
Historically, the Senate is slightly better for the president's party.
Since World War II they've gained ground four times in midterms, held their ground twice – but still lost seats on 13 occasions.
What factors are influencing the midterm vote?
In a non-compulsory voting system like America's, elections hinge on who turns up to vote.
Often supporters of the out-of-power party get motivated to cast a ballot, while supporters of the party in office can become disillusioned and just stay home.
President Biden is facing a potential backlash for decisions such as pushing ahead with the agreement to end the US presence in Afghanistan brokered by his predecessor Donald Trump, clearing the way for a return of the Taliban.
Biden has also signed into law several billion dollars' worth of spending on COVID-19 relief, infrastructure, and climate change measures which some believe helped fuel inflation.
On the other hand, outrage at the June decision of the US Supreme Court to overturn Roe v Wade – the constitutional right to abortion access in the first trimester – may see Biden's Democrats defy history.
Biden's personal approval has rebounded nearly five points since the ruling, and the Republicans have lost their two-point lead in the average of generic congressional polls – the Democrats are now 1.5 points ahead.
Is Trump still a key player?
Another factor may be the Republicans' choice of candidates.
In several key contests which could decide the senate majority, unconventional and inexperienced Trump-backed Republicans – like former footballer Herschel Walker in Georgia, and TV doctor Mehmet Oz in Pennsylvania – are struggling in races they were expected to win.
These elections are as much a referendum on Donald Trump as they are on Joe Biden – and could decide whether a Trump-Biden rematch is on the cards in 2024.
What else do I need to know?
There are also elections for governors and other office holders in thirty-six American states and three territories this November.
Those contests could have a significant impact on future presidential races, with Democratic governors such as California's Gavin Newsom and Michigan's Gretchen Whitmer pondering White House bids.
Nevertheless, there is good news for President Biden.
History also suggests a defeat for Democrats won't necessarily affect his chances of winning a second term if he runs in 2024.
In the last century, almost twice as many sitting presidents who stood for election, won rather than lost, and all but two of them had suffered midterm losses along the way.
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