LONDON — Britain’s voters go to the polls Thursday faced with a stark choice between Boris Johnson’s Conservative Party, which is campaigning on a message to get Brexit done, or Jeremy Corbyn’s hard-line left Labour Party. Johnson has what could be a decisive lead in the opinion polls, but tactical voting and the country’s first-past-the-post electoral system, which gives the win to the candidate who gains a plurality of the vote, means there could be surprises when votes are tallied.

Polls close at 10 p.m. local time (5 p.m. ET). The first indication of who will form the next government comes at the same time, with the publication of an exit poll commissioned by broadcasters.

The first constituencies are expected to declare their results an hour later. But the full picture will not emerge until the early morning (around 3 a.m.) as the main target seats announce their results.

Here’s what to watch for:

Turnout

General election turnout has been increasing in the U.K. since 2001, and reports of queues at polling stations suggest high numbers of people are casting their vote this time around. However, this is the first election held in December since 1923, a month when poor weather and fewer than eight hours of daylight could keep voters at home.

A third election in four years and the bad-tempered Brexit debate have also generated a general air of weariness.

A lower turnout is predicted to help the Conservatives, whose older supporters are more likely to cast their vote than younger Labour supporters. But if the turnout is high, then it could mean better news for Corbyn.

Tory targets

If Johnson is to win a majority in Parliament, the Conservatives need to win at least 326 of the 650 seats. He has staked his job on getting Brexit done, which means he probably needs a majority of 16 or so to pass the crucial vote without having to worry about a handful of his MPs acting up.

The first signs of whether he is on course will come at about 1:30 a.m., when Workington and Darlington, two long-held Labour seats, are expected to declare their results. These should be parts of Labour’s northern “red wall,” but if they fall, then it means Johnson’s “get Brexit done” strategy is resonating with blue-collar voters.

Big beasts in danger

Every election has its share of big-name casualties. Their defeats often provide the defining moments of the night.

Even Johnson himself carries some risk, with a majority of only 5000 or so in his constituency. But the biggest danger could be to Foreign Secretary Dominic Raab. His 23,000 majority should make his seat as safe as they come, but his ardently pro-Brexit position is at odds with local voters who plumped 58% for Remain, making him vulnerable if Labour supporters lend their ballots to his Liberal Democrat challenger.

Iain Duncan Smith, a former party leader, and Zac Goldsmith are other big names who are at risk from tactical voting.

Scottish independence

Scotland voted against independence in 2014. However, Scottish nationalists hope that an overwhelming victory in the general election could strengthen their case for a second referendum.

The Scottish Nationalist Party holds 35 seats, and one eve-of-election poll suggested they could be on course to have as many as 41 MPs in the next Parliament. Their ability to demand a referendum on independence would be further strengthened in the event of a hung Parliament with no overall winner when their support could prove crucial to one of the bigger parties.

The value of the pound

There’s generally one thing that bankers want from an election: stability. And the pound has climbed against the dollar over the past two months, from $1.22 to $1.31 as poll after poll suggested a Johnson majority was much more likely than the political chaos of a hung Parliament.

The pound initially rose on Thursday before falling back. Keep an eye on markets as the results become clear.

[Opinion: Will Britain's election tomorrow deliver a Boris Brexit bonanza or Jeremy Corbyn's Soviet Britannia?]