LONDON — The final hours of campaigning have begun ahead of Thursday’s election, which is being called the most fractious vote in the United Kingdom’s recent history with the outcome of Brexit still in the balance. This, the 19th general election of Queen Elizabeth II’s reign, could turn out to be the most important.

Why does it matter?

While Labour, the main opposition party, and others have been keen to emphasize domestic issues such as public health service, school funding and poor transport links outside of London, this will still be remembered as the Brexit election. The split between leave and remain voters represents a cultural fault line in British society — with anger and dissatisfaction with politics expressed in the 2016 vote spilling over into this election.

“The Brexit referendum was a moment of real change in British politics but it didn’t come out of nowhere,” Simon Usherwood, a professor of politics at the University of Surrey, said.

“It’s about the difference between the people who feel the system has worked for them, who have benefited from economic modernization and globalization, and those people who haven’t,” he added. “We keep on using the term in the U.K. the ‘left behind’ — Brexit was an opportunity for them to voice their discontent, their disconnection from the system.”

There are fears that campaigns for Brexit, many of which drew on discontent with mass migration to the U.K., unleashed dangerous forces in the country. In fact, there was a 41 percent spike in hate crimes, according to government statistics, in the month following the Brexit vote.

Thursday’s election — the third in four years — was called by Parliament because Prime Minister Boris Johnson failed to get his Brexit deal through the House of Commons. Johnson secured the divorce deal with the remaining 27 European Union states in October, but U.K. lawmakers get the final say and a majority voted against it.

The U.K. has now missed three deadlines to leave the E.U. — missing its fourth Jan. 31, even with a solid win Thursday, could well spell the end of Johnson’s premiership.

Who’s in power?

Polls have consistently showed Boris Johnson’s ruling Conservative Party with a lead against left-wing Labour, which would allow him to form the next government and fulfil his main campaign promise to “get Brexit done.”

The Conservatives, known as the Tories, have been in power since 2010. Traditionally, the party represents the interests of business, favors a small welfare state and likes to offer tax cuts to aspiring middle-class voters. Under Johnson, the party has partly abandoning the stringent fiscal rules in an attempt to distance itself from the much-hated period of austerity of his predecessors.

Johnson, who campaigned for Brexit, is popular among Conservative voters, although he has been linked to a string of controversies in his personal life and faced some awkward encounters with voters.

The U.K. Supreme Court ruled that he effectively misled the Queen on his reasons for suspending Parliament — Johnson claimed it was to push through a domestic policy program, but the court said it was an attempt to frustrate democracy and stop lawmakers from scrutinizing his Brexit plans.

He is nevertheless favorite to remain in 10 Downing Street.

What could happen after Dec. 12?

If recent polls are accurate, the most likely outcome is a Conservative majority.

But if that doesn’t happen, the government and voters could face what is known as a hung Parliament, which is when no party or existing coalition wins a majority of the 650 seats being contested.

“The Conservatives are ahead of Labour in the polls but this is really an election between the Conservatives and everybody else,” Rob Ford, professor of politics at Manchester University, said.

It’s also possible that if they fall short, the Conservatives could try to form a coalition, as they did with the Liberal Democrats in 2010. A far less likely outcome is a coalition led by Labour, perhaps propped up by the Scottish National Party.

Who are the players?

Boris Johnson, who went to the elite private school Eton and Oxford University — a well-worn path for the country’s ruling class — came to notoriety as a journalist and an occasional guest on satirical TV quiz shows.

He has faced many questions about his character and honesty, and was fired from the Times of London in 1987 for inventing a quote. Johnson was later fired as a senior Conservative spokesman in 2004 for lying about an affair.

Labour is led by Jeremy Corbyn, long an outlier in his party who rejected the center-left policies of recent leaders in favor of a socialist agenda. While popular with the party’s rank-and-file, he is very unpopular among many colleagues in Parliament who have tried and failed to oust him.

The party is promising a radical overhaul of the U.K. economy by taking energy and transport companies, post offices and parts of the main telecom provider into public ownership.

Under Corbyn, the party’s membership swelled to 485,000, compared to 180,000 Conservative members.

However, Corbyn has also been accused of mishandling accusations of anti-Semitism from some Labour activists, an issue that has overshadowed the campaign.

In September a poll found Corbyn was the least popular opposition party leader for 45 years, with just 16 percent of voters pleased with him and 76 percent unhappy. And while his poll ratings have grown throughout the campaign, in Labour heartland constituencies in the north of England, where many people voted to leave the E.U, voters are particularly disillusioned with the party.

The Liberal Democrat Party, which is standing on an anti-Brexit platform in the hope of capturing voters who want to remain in the E.U., started the campaign well but is now expected to win 11 percent of the total vote share, according to a poll Monday from Survation, and will hope to improve on its current 21 seats.

Its leader, Jo Swinson, 39, was the youngest member of Parliament in the House of Commons when first elected in 2005 and served as business minister in the Conservative-Liberal Democrat coalition from 2012 to 2015.

The Lib Dems, as they are universally known, managed to persuade several high-profile lawmakers from the Conservatives and Labour to defect and for a time it appeared that the party would win enough seats to play a role in the next government.

But the party has seen its poll rating fall throughout the campaign, partly due to its divisive policy of canceling Brexit for good if it came to power.

The Brexit Party, founded by former U.K. Independence Party leader Nigel Farage, a populist admirer and ally of President Donald Trump, started the election campaign with the hope of winning several seats.

Then, despite having heavily criticized Johnson’s Brexit deal, the party decided not to run candidates in hundreds of seats being contested by the Conservatives and told voters to back Johnson’s party instead.

The party is now only polling at around 4 percent of the total vote share.

Then, there is the Scottish National Party wants Scotland to be an independent nation separate to the U.K. Its leader, Nicola Sturgeon, is a passionate anti-Brexit campaigner, reflecting the views of the majority of Scots, who voted to remain in the E.U.

Sturgeon is the first minister of Scotland, meaning she leads the Scottish Parliament — the U.K. has a system in which Scotland, Wales and Northern Ireland make some of their own laws.