The election could go one of three ways. What would each result mean?

After five weeks of campaigning, the Tories are ahead of Labour in the polls – but few experts dare predict the result. The surprise outcomes of the 2016 Brexit referendum and 2017 general election stung pollsters and pundits alike.

Last week, the polling expert Sir John Curtice said Boris Johnson was by no means home and dry, pointing out that if Labour could increase its share of Remain voters by polling day it could yet spring a surprise.

Broadly, he and others will go no further than to say there are three possible outcomes: a Tory majority, another hung parliament, or (by far the least likely at this stage) a Labour majority.

So what will each of these mean for the party leaders, for Brexit, the economy and government spending, and for the union of England, Scotland, Wales and Northern Ireland?

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A Tory majority

Party leaders

Johnson will be secure in No 10 for the foreseeable future, at least until another Brexit crisis erupts (which could happen by the end of next year). But Jeremy Corbyn, having failed to beat the Tories for a second time, will almost certainly be on the way out. The big question will be when. Will he stay on for a short time to allow a “period of reflection” as Len McCluskey, the boss of Unite, suggests?

At some point before the autumn a succession battle will ensue between candidates from the Corbynite left, soft left and moderate wings. If the Lib Dem leader, Jo Swinson, fails to win more seats for her party, or loses some, she could be in trouble too, though most Lib Dems think she will cling on (assuming she wins her own seat) so long as her party avoids a total flop.

Brexit

Johnson will get his deal through parliament and the UK will leave the EU on 31 January. But that will not be the end of the Brexit saga. The UK will then enter the transition period, during which it must agree a trade deal with the EU to avoid leaving with no agreement at the end of 2020.

If Johnson wants to extend the transition he can ask the EU, but he must do so by the end of June next year. He has said he will not ask for an extension under any circumstances. But trade deals usually take several years to negotiate and ratify, not a few months. A Johnson government could be in another full-blown Brexit crisis – with familiar arguments raging about the merits of a further extension and the danger of a no-deal exit – very soon.

The economy/tax/spending

One key Tory campaign message has been that austerity is now over. Johnson’s Tories will loosen the purse strings. The chancellor, Sajid Javid, has promised a £13.8bn spending surge across departments by 2021, with more for the NHS, schools and police.

Johnson has pledged to cut national insurance, business rates, construction tax and research and development tax. A planned 2% reduction in corporation tax has been put off to pay for extra NHS funding. But the Institute for Fiscal Studies says the Tory plans do not add up.

The future of the union

Brexit has already fired up the debate about Scottish independence. If the Tories form a majority government – and the anti-Brexit SNP do well north of the border – the pressure for a second referendum will intensify. Scotland’s first minister, Nicola Sturgeon, says her country’s opposition to Brexit has reinforced the case for independence and she wants another referendum either in 2020 or 2021.

Johnson’s Brexit deal, which would establish a border between Northern Ireland and the rest of the UK, has also led to calls from Irish nationalists for a referendum on unification of the north and south of the island.

A hung parliament

Party leaders

If the Tories fail to win a majority, Johnson will be down but not necessarily out. As the sitting prime minister (assuming he does not resign) he will get the first shot at forming a minority government, whether his party has the most seats or not. But finding partners to prop him up will be difficult.

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The DUP detest Johnson’s Brexit deal and the Lib Dems say they won’t go near the Tories, not least because they want to reverse Brexit, and because they are still recovering from the 2010-15 Tory/Lib Dem coalition. If Johnson fails to form a government and Labour does, he will be in big trouble. If Labour gets its chance, it could team up with the SNP and agree to back a second independence referendum, two years or more down the line. The Lib Dems have said they would not think of propping up a Labour government if it were led by Corbyn. So he may be pressured to step aside.

Brexit

A hung parliament is likely to halt Johnson’s plan to leave the EU on 31 January and could open a route to the UK staying in the EU. Without a majority, and with the DUP opposed, it is hard to see how he could get his Brexit deal through parliament, unless he agreed to make approval of it subject to a second referendum, with the other option being to remain in the EU. This could be his only hope. If a Labour minority government were formed then Labour, the Lib Dems and the SNP would all favour a second referendum as the way to break the impasse, though the details of what question would be asked would be the subject of difficult negotiation.

The economy/tax/spending

Money will be at the heart of negotiations between parties haggling over arrangements for a minority government. And the spending plans of both the Tories and Labour as revealed during the campaign will be unlikely to survive any deal-making. After the 2017 election the DUP struck a hard bargain as the price of its support for Theresa May’s government, winning £1bn more for Northern Ireland in return for its participation in a “confidence and supply” arrangement. If the Lib Dems or SNP were to back a Labour-led government, both would demand a financial price.

The future of the union

A hung parliament could advance or damage the cause of Scottish independence. If more deadlock in the Commons leads to a second Brexit referendum and a Remain vote meaning the UK stays in the EU, support for independence could dwindle somewhat. But if the SNP can act as kingmakers for a Labour minority government, they would only do so with the guarantee that Labour would back another independence vote. Jeremy Corbyn has said he would not support another Scottish independence referendum for at least two years, suggesting he could do so after that.

A Labour majority

Party leaders

If Labour wins outright, Corbyn will be installed in Downing Street, probably until another general election in five years’ time, unless he decides to hand over earlier. But Johnson will almost certainly be out after just five months in No 10, and the Conservative party will be plunged into a huge battle over its future direction. Swinson’s future will depend on how well or badly her party has performed, but a poor performance that has contributed to a Corbyn government will be hard for Lib Dems to stomach.

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Brexit

Labour will try to renegotiate its own Brexit deal with Brussels and then make that the subject of a second referendum, with the other option being to remain. Corbyn has said he will stay neutral in those circumstances, though members of his shadow cabinet are likely to be given the freedom to campaign as they wish. Experts believe Labour will struggle to negotiate a new deal and hold a referendum within six months, but if it wins power it will not worry too much if that timetable has to be stretched. Labour has promised that the result of a second referendum would be legally binding, meaning parliament would have to enforce it.

The economy/tax/spending

Labour has promised to “rewrite the rules of the economy” with huge investment in public services, to be paid for, it says, by imposing higher taxes on the better-off. People earning more than £125,000 would have to pay a new 50p rate of income tax and the threshold for the 45p rate would be lowered from £150,000 to £80,000. Labour would also take the water industry, energy companies, the Royal Mail and railways under state control. The Institute for Fiscal Studies has said it would be “highly likely” that Labour would have to raise taxes beyond what it has announced to pay for its proposed £80bn a year of extra spending.

The future of the union

If a second referendum under Labour still delivers Brexit then the pressure for a second independence referendum will remain intense. But if the UK remains in the EU it could subside for the time being. Similarly, if Labour delivers a different Brexit deal without a customs border between Northern Ireland and the rest of the UK – or a referendum and a Remain outcome – then the Brexit-induced tensions on the island of Ireland should also be reduced, lessening the renewed clamour from nationalists for unification of north and south.