Britain is set for a dramatic election night Thursday. Will Conservative Prime Minister Boris Johnson land the parliamentary majority he needs to secure his Brexit deal?

According to the latest opinion polls the Conservatives seem likely to win a majority. Here are the four factors likely to shape the outcome:

1. Johnson wants a Brexit focus; Labour wants to change the subject

In interviews and debates, Johnson has repeated his central slogan and promise to “Get Brexit Done.” The message appeals to those who voted in June 2016 for Britain to leave the European Union, while speaking to the general public’s disillusionment and frustration with politicians who have bickered over Brexit for the past three and a half years.

The Conservatives’ chances of becoming the largest party in Parliament got a boost when Nigel Farage’s Brexit party decided not to run in seats with a Conservative incumbent. However, research suggests the Brexit party takes more voters away from the Conservatives than Labour. So in districts where Brexit party candidates are running, Conservatives may find it tough to win seats held by Labour.

The opposition Labour Party has tried to move the debate away from Brexit, and instead emphasize health care and wider public services. Britain’s National Health Service, free at the point of need, is highly popular among British voters and indeed is as important to them as Brexit. Labour leader Jeremy Corbyn has warned voters the Conservatives will privatize the health system and open the NHS to powerful U.S. pharmaceutical companies in any post-Brexit trade deal.

Labour’s manifesto promises a massive 150 billion pound ($197.5 billion) investment in public services, blaming the Conservatives and their austerity measures over the past decade for the poor state of public services in Britain. Although he was foreign secretary in Theresa May’s government, Boris Johnson has been keen to distance himself from the record of the Conservatives, stressing his “new” government has only been in power for just over 100 days. Johnson has tried to counter Labour claims by promising more nurses and more hospitals — though these messages have been muddy and have come under sustained attack from opposition parties.

2. The Conservatives want to reshape Britain’s political geography

A key plank of Johnson’s strategy is to break the “Red Wall” — a zone spanning North Wales, the Midlands and Northern England that includes Labour’s traditional heartland seats. Many working-class Labour voters across this region strongly supported Brexit three years ago, and Johnson is gambling that these voters will now abandon their usual partisan attachment and prioritize Brexit.

Support for Labour in these seats has been falling. Voters feel “left behind” economically and believe the Labour Party’s national leaders tend to ignore this region’s socially conservative cultural concerns. Labour is hoping that a combination of partisan loyalty and worries about public services will keep them afloat.

The British Election Study from October showed that more people changed their vote in the last two elections than ever before. In fact, nearly half of British voters voted for different parties across the last three general elections.

If traditional Labour strongholds become this volatile, then Labour could be in trouble. Some of the first seats to declare Thursday night will be in the Red Wall so it’s likely we’ll know fairly early just how effective Johnson’s strategy has been.

3. Tactical voting may make a difference

While Labour is facing a tough job fending off the Conservatives in their Red Wall seats, tactical voting in parts of the country that voted to “remain” in the European Union could make it harder for Johnson to win a majority.

Remain campaigners have encouraged their supporters to vote for the candidate best placed to defeat the Conservative. Some prominent Conservative politicians look vulnerable — Foreign Secretary Dominic Raab, former party leader Iain Duncan-Smith and even Johnson himself. Johnson may lose his seat of Uxbridge and South Ruislip, a district on the edge of London with a significant student vote, if there is just a 5 percent swing from Conservatives to Labour.

Tactical voting is not new, but Brexit has raised the stakes for voters who know that a Conservative majority will mean Britain will leave the European Union on Jan. 31, 2020. However, the various tactical voting websites offer conflicting advice in key marginal seats. Since Labour and the Liberal Democrats are fighting each other to be the “remain” alternative in some of these seats, the anti-Conservative vote could split, allowing Johnson to minimize his losses.

4. Turnout might be the key variable

Getting people out to vote could swing the balance in a few close contests. In the last British general election, in June 2017, 11 seats were decided by 100 votes or fewer. And 97 seats were won by 5 percent or less of the votes cast.

The Conservatives rely more on the air campaign, including Facebook and video ads to mobilize potential supporters in key contests. But Labour may have an advantage in the ground game, thanks to its larger activist base. Here’s the question — will Labour strike the right balance between mobilizing their voters to defend vulnerable Red Wall seats and encouraging voters to turn out for Labour in winnable seats in Remain areas?

The Conservatives will be hoping that disillusionment with the Brexit impasse may mobilize voters who turned out to vote “leave” in 2016 but didn’t vote one year later in the general election. Labour hopes that the surge in voter registration, particularly among voters under 25, will tip the balance in seats with large student populations.

In the end, the British weather may play an important role. If the forecasts of heavy rain for Thursday are accurate, many voters may simply feel unenthusiastic about grabbing coats and umbrellas on a wet December day to cast their ballots.

David Cutts is professor of political science at the University of Birmingham.

Tim Haughton is associate professor of political science at the University of Birmingham.