LONDON — U.K. voters go to the polls on Thursday for their third national election in five years. At stake is the future direction of Brexit, and even whether Britain's departure from the EU happens at all.

The five-week campaign ahead of the highly unusual December election (the first since 1910) has also revealed stark differences between the two main parties over how the country should be governed.

The Tories have held a consistent lead throughout the campaign, with Labour's leader Jeremy Corbyn highly unpopular with voters, according to the polls. Nonetheless, Remain supporters hope that tactical voting in a handful of key constituencies can deny Prime Minister Boris Johnson a majority and keep alive the dream of a second Brexit referendum.

The POLITICO team will be live-blogging throughout election day and night. Here’s our rundown of what to look out for:

How it works

General elections in the U.K. use a "winner takes all" system. Under the first-past-the-post electoral system, MPs each represent one of 650 constituencies which gives them a seat in the House of Commons. To become prime minister and form a government, a party needs to win more than half the seats in the House of Commons (though the exact number is lower than half the total because of Sinn Féin MPs not taking their seats plus non-voting members such as the speaker and his deputies). If all parties fall short, Britain has a hung parliament and deals must be done to form a government. When parliament was dissolved for the election last month, Johnson was 19 seats short of a majority (taking into account Sinn Féin MPs, vacant seats and the speaker).

Scenarios

A great night for Johnson: A majority of 50 or above would mean that Johnson’s election gamble has paid off handsomely. It should allow him to pass his Brexit deal comfortably by January 31, allowing his government to begin negotiations with the EU on a post-departure trade deal. Last month YouGov’s MRP model, which was the first to predict a hung parliament in 2017, forecasted a 68-seat majority for the Conservatives. But a new poll on Tuesday gave the Tories a 28-seat majority.

A win, but only just: A majority of 15 MPs or fewer would allow Johnson to press ahead with his plans, but make him vulnerable to rebellions from backbench MPs. All Tory MPs are signed up to the deal Johnson agreed with the EU in October but a coordinated effort by Brexiteers on the right of the party could force him in the direction of a harder position in the next phase of negotiations. Theresa May called the 2017 election because she thought the 12-seat majority won by David Cameron in 2015 was too small to pass a Brexit deal.

Hung parliament: A hung parliament would represent a humiliation for Johnson after weeks of leading in the polls. If the Conservatives won the most seats they would get the first chance to form a government, but doing so would be tricky — their erstwhile allies, the Democratic Unionist Party, are refusing to back the existing Brexit deal. Labour, the Liberal Democrats and the Scottish National Party would fancy their chances of passing legislation for a second Brexit referendum, which many MPs would then consider the only way to end the deadlock.

Labour majority: Pollster John Curtice has said the chances of a Labour majority are as “as close to zero as one can safely say it to be,” in part because the party looks exceedingly unlikely to win back the 59 Scottish constituencies it used to hold before 2015. But if by some miracle Labour did win a majority, it would be in a strong position to implement its policy of negotiating a new Brexit deal and putting that to a new referendum within six months — as well as its radical economic agenda. Such a crushing reverse for the Tories would also force Johnson's resignation as Tory leader.

Election day

Voting begins at 7 a.m. and will keep going until 10 p.m. local time. British news coverage will be fairly election-free during the day with reporting restrictions preventing coverage that indicates how the vote is going while the polls are open, meaning journalists are restricted from reporting much more than politicians going to cast their votes and that voters are going to the polls. POLITICO's election live blog will run from 7 a.m. and continue through the night to Friday afternoon. For email alerts on key developments through the night sign up for POLITICO's Election Sprint.

10 p.m.

The national exit poll drops. The poll is commissioned jointly by the BBC, ITV and Sky News, with the fieldwork this year conducted by Ipsos MORI. Its findings are based on tens of thousands of interviews outside 144 polling stations with people who have just cast their vote — so it is considered more accurate than mid-campaign polls that have to factor in respondents' likelihood to vote. Professor John Curtice of Strathclyde University will lead a team of academics to analyze the results.

The exit poll tends to give a fairly accurate indication of the result. In 2017, it correctly forecast that the Conservatives would lose their majority. In 2015 it found that the Tories would be the largest party — though failing to predict their majority — and in 2010 it pointed to a hung parliament.

11 p.m.

Shortly after 11 p.m., all eyes will turn to the northeast, where there is a race between the people of Sunderland and Newcastle to be the fastest to declare their result.

Houghton and Sunderland South (and its predecessor prior to boundary changes, Sunderland South) was the first seat to declare in six consecutive elections, but in 2017 it was pipped by Newcastle upon Tyne Central by seven minutes.

Other nearby constituencies are also set to declare very early on. Keep an eye on whether there is a marked swing to the Conservatives in these otherwise safe Labour seats. It could provide an early indication of whether Boris Johnson has succeeded in breaking Labour’s hold on the north of England.

1 a.m.

News will pick up as a flurry of results start coming in, including some key marginal seats. One to watch is Labour-held Workington, which attracted media attention after center-right think tank Onward concluded that the “Workington man” was the archetypal target voter the Tories must win round.

Rutherglen and Hamilton West, the first Scottish constituency expected to declare, will be an indication of what’s going on across the border. It was won by Labour in 2017, with the Scottish National Party snapping close at its heels. If the SNP takes it back, we could be looking at a bad night for Labour in Scotland.

Wrexham could declare around now if the 2017 election is a guide — one of the first in Wales to do so. Labour has a new candidate and is defending a majority of less than 2,000. If the Conservatives can't win here — the area voted solidly in favor of Leave in the 2016 referendum — then it is going to be a long night for Johnson.

1:30 a.m.

Key bellwether seat Nuneaton is set to declare around now. Whichever party takes this constituency tends to win the election. In 2015, David Cameron pinpointed it as the moment he knew he’d won.

The result in Darlington, a Brexit-voting Labour seat and key Tory target, is also expected before 2 a.m.

Seats that declare early on tend to be Labour-voting ones — but that’s not necessarily an indication of what is happening nationwide.

2 a.m.

Things start to get frantic, as the bulk of constituencies prepare to declare between 2 a.m. and 4 a.m.

Key battlegrounds include Labour-held Battersea and Bury North and Tory-held Putney in London and Thurrock in Essex. The result in Birkenhead in the North West, where veteran former Labour MP Frank Field is standing for the first time as an independent, will also come around 2 a.m.

Keep a weather eye on West Bromwich East in the Midlands. In the last parliament it was held by Labour deputy leader Tom Watson before his shock announcement that he would not stand for reelection. It would be a major scalp for the Conservatives if they can take the seat. On the other hand, Steve Baker — chair of the backbench group of Tory Brexiteers, the European Research Group — is defending a majority of just 6,500 in Wycombe.

Another one to watch is Hartlepool, which has been Labour since its creation and was formerly held by ex-Cabinet minister and EU Trade Commissioner Peter Mandelson. The seat is unofficially the top target for the Brexit Party, which has put up its chairman and MEP for the east of England Richard Tice as the candidate.

3 a.m.

Results will continue to come in thick and fast. Around 3 a.m. the result in the South East seat of Esher and Walton, held by Foreign Secretary Dominic Raab, is expected. The Liberal Democrats say they have a good chance of unseating him — but the Conservatives are playing down the prospect of an upset.

Another high-profile Tory, ex-leader Iain Duncan Smith, is defending a slim majority in Chingford and Woodford Green in east London from a massive campaign push from Labour's grassroots Momentum group.

Around the same time, Liberal Democrat Jo Swinson will find out her fate in East Dunbartonshire. She lost her seat in the 2015 election but won it back with a 7-point swing to her party from the SNP, giving her a majority of over 5,000. Enough Labour supporters voting tactically for the SNP could oust the Lib Dem leader. Watch out also for the result in the U.K.’s most marginal constituency, North East Fife, which was won by the SNP in 2017 with a majority of just two.

Another big hitter at risk is the Democratic Unionist Party's Westminster leader Nigel Dodds in Belfast North. The Social Democratic and Labour Party have stood aside here to help the Sinn Féin candidate. Losing Dodds would be a massive blow to the DUP. The party could also lose Belfast East and Belfast South, whose results are expected around the same time.

Other seats of interest are Beaconsfield in the South East, where former Tory MP Dominic Grieve is standing as an independent — the Lib Dem candidate stood aside to help him because of Grieve's Remain credentials — and Sheffield Hallam, the first major Lib Dem target seat of the night. They hope to take the seat from Labour.

The Cities of London and Westminster is being contested by former Labour MP Chuka Umunna, who switched to the Liberal Democrats earlier this year. He would need to come from a distant third to clinch the seat from the Tories.

A result is also expected from the top Green Party target of the Isle of Wight, held by the Conservatives with a healthy majority of over 20,000. In 2017, the Green candidate came third behind Labour with almost 13,000 votes, and this time the Lib Dems have stepped aside to help her.

The result in Jeremy Corbyn’s Islington North is expected around 3 a.m., and the cameras will turn to his speech: triumphant if his party is making gains and subdued if it is not.

3:30 a.m.

We should hear from key Labour targets Aberconwy in North Wales and Hastings and Rye on the south coast — both held by the Conservatives with majorities of 635 and 346, respectively — at this point in the evening. In both cases, the incumbent Tory MP is standing down, but even so, if Labour makes gains here then Johnson's dreams of a majority will be evaporating.

Two high-profile Lib Dems, former leader Tim Farron and ex-Tory convert Sarah Wollaston, will also find out around now whether they have kept their seats. In 2017, Farron came close to losing his Westmorland and Lonsdale seat in the North West of England, scraping through with a majority of 777 over the Conservatives. The Greens are standing aside to help him retain the seat as part of the Remain alliance. Wollaston is defending a majority of over 13,000 in Totnes in the South West, but it remains to be seen how voters will react to her switch of party.

4 a.m.

Results will come in from lots of the marginals Labour won from the Tories in 2017, including Canterbury and Croydon Central in the South East, Ipswich in the East of England and Crewe and Nantwich in the North West. Johnson will need to take back seats like these to achieve a majority in the Commons.

The Lib Dems are vying to overturn a 2,569 majority for the Tories in Cheltenham and have assistance from the Greens who are standing aside. Two Cabinet ministers with majorities of below 3,000 votes, Robert Buckland (South Swindon) and Alok Sharma (Reading West), will also find out their fate. Prominent Brexiteer and Leader of the House Jacob Rees-Mogg in North East Somerset looks safer with a majority over 10,000.

4:30 a.m.

By now, Boris Johnson will have headed to his constituency of Uxbridge and South Ruislip for the result of his count, expected by 4:30 a.m.

Johnson is defending a majority of 5,000 and could conceivably lose the seat, which has been targeted very heavily by Labour and Momentum activists. Either way, there will be some great photos of the prime minister standing next to his fellow parliamentary candidates Lord Buckethead and Count Binface.

All eyes will be on the result in Kensington, where the Tories hope to unseat Labour’s Emma Dent Coad, who took the seat in a shock victory in 2017 with a majority of just 20.

5 a.m.

Lots more Labour target seats like Southampton Itchen on the South coast (Tory majority 31) will be declaring, as well as Lib Dem targets Cheadle in Greater Manchester and Hazel Grove in the North East. In both cases the Greens are standing aside. The Tories will be hoping to take Ashfield in the Midlands off Labour where the party is defending a 441 majority and where prominent MP Gloria de Piero has opted not to stand again.

Look out for Bolsover in the East Midlands, where veteran Labour MP and ex-miner Dennis Skinner is seeking reelection — but is under threat from the Tories. If Skinner wins, he will be the new Father of the House (the longest continuously-serving male MP).

Former Tory Justice Minister David Gauke will find out whether he has been reelected as an independent in South West Hertfordshire. As a Tory, he was sitting on a massive majority of nearly 20,000, but many of those votes may move to that party's new candidate. In SNP-held Ross, Skye and Lochaber, its Westminster leader Ian Blackford could be under threat from the Lib Dems because of tactical voting. He has a majority of just under 6,000 but the seat was a Lib Dem stronghold up until 2010.

Another interesting contest that will declare around now is Broxtowe in the Midlands. This is a Tory-Labour marginal with fewer than 1,000 votes separating the two. But incumbent pro-Remain Tory MP Anna Soubry resigned from the party in February and is now the leader of the Independent Group for Change.

5:30 a.m.

Still awake? Environment Secretary Theresa Villiers will find out whether she has been unseated in Chipping Barnet in North London, where she is defending a tiny 353 majority against Labour.

A late result will come from another Tory-held North London seat, Finchley and Golders Green, which is being targeted both by Labour and the Lib Dems. Labour were just 1,657 votes behind in 2017 but in a seat with a large Jewish population, their candidate may struggle because of the anti-Semitism scandal engulfing Jeremy Corbyn's leadership. The constituency had a "Love Actually" moment during the campaign when actor Hugh Grant went knocking on doors with Lib Dem candidate Luciana Berger — herself a defector from Labour.

6 a.m. and later

In Richmond Park in West London, the Liberal Democrats look in a strong position to take back the seat from Brexiteer Zac Goldsmith. Their candidate, Sarah Olney, took the seat in a by-election in 2016, but Goldsmith won it back in 2017 with a majority of just 45. Later in the morning, results will come in from key Lib Dem target seats in the South West of England, including North Cornwall and St. Ives. In both cases they are up against the Tories.

The result in Brighton Pavilion, the only seat held by the Green Party, should come in around 7 a.m. The Liberal Democrats have stepped aside to support the party's former leader Caroline Lucas.

Who's won?

Results will keep coming in until breakfast time, but the winner should be clear by the time most people’s alarms have gone off.

Television

For half a century, the BBC's election night has been a family affair, presented between 1979 and 2017 by David Dimbleby and before that by his father Richard Dimbleby. This year, the national broadcaster's coverage will be fronted for the first time by Huw Edwards. Polling guru John Curtice, however, will remain a fixture.

Sky has booked the rambunctious former House of Commons Speaker John Bercow as a special guest on its election show hosted by Dermot Murnaghan. A separate live program developed in partnership with BuzzFeed and streamed on social media will be presented by Lewis Goodall.

On ITV, viewers will get to watch the first broadcast interview with Theresa May’s former chief of staff Fiona Hill since the 2017 election. Guests on the broadcaster’s election panel will once again include former Chancellor George Osborne and his Labour counterpart Ed Balls. Boris Johnson’s brother Jo, who quit the Cabinet earlier this year over the direction of Brexit, will also take part.

The morning after

Victory speeches: If there is an outright winner, we are likely to have a victory speech early in the morning. Questions about the future of the leaders of the defeated parties will surface quickly. In 2015, Ed Miliband and Nick Clegg — the leaders of Labour and the Liberal Democrats, respectively — both resigned the day after the election.

The Queen: If Boris Johnson has lost definitively, he will step down and the queen will invite the person most likely to command the confidence of the House of Commons to form a government. In the event of a hung parliament, where no party has an overall majority, she waits for political parties to determine who is best-placed to govern.

Hung parliament: In this scenario the Liberal Democrats and Scottish National Party are likely to find themselves in a kingmaker’s position. The party that has won the most seats will have the first chance at forming either a coalition or a looser confidence-and-supply agreement.

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