General Election 2015: The three most likely outcomes

The scenarios that will determine whether Cameron or Miliband is PM. 

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Tomorrow, parliament will be hung again. That is the only prediction that most make with confidence about the election. For the first time since 1910, voters will deny any party a majority for a second successive contest. All of the final forecasts show Labour and the Conservatives falling far short of the 326 seats they need for overall victory. But a hung parliament encompasses a range of possible outcomes. Below are the three most likely.

1. The Tories are the largest party but cannot form a majority with other parties

Almost all of the major forecasters predict the Conservatives will remain the single largest party, with Labour badly hurt by the loss of most of its 40 Scottish seats (the outcome also privately expected by the two main parties). But most also suggest that even with the support of the Liberal Democrats and the DUP, the Tories would fail to achieve the 323 seats they need for a de facto majority in the Commons (excluding the Speaker and the five abstentionist Sinn Fein MPs likely to be returned).

In these circumstances, Labour would demand that David Cameron resign as prime minister, citing paragraph 2.12 of the Cabinet Manual, which states that "An incumbent government is entitled to wait until the new Parliament has met to see if it can command the confidence of the House of Commons, but is expected to resign if it becomes clear that it is unlikely to be able to command that confidence and there is a clear alternative." 

But in recent days the Conservatives have suggested that they would proceed to a vote on the Queen's Speech even with little prospect of winning over MPs. The aim is to ensure that a putative Labour administration takes power in the most difficult circumstances, with the Tories framing an Ed Miliband government as "illegitimate" on the grounds that his party finished second and would depend on the support of the SNP. As one Conservative cabinet minister told the Sunday Times: "It’s a race to be the largest party. We will say: 'We’re legitimate, we’re the largest party, we should carry on.' If necessary, dare the others to vote down a Conservative government. We’ll bring forward a vote of confidence on our Queen’s Speech so they do the deed in plain sight, rather than meekly saying, 'I suppose your numbers add up, goodbye'." The most optimistic Tories hope that they could persuade some Labour MPs and others to abstain in these circumstances, allowing them to remain in power even without the 323 seats required for a majority. 

2. The Tories are the largest party and can form a majority with other parties

If the Conservatives are able to make it to 323-plus seats with the support of other parties (winning around 290 themselves) then work will immediately begin on a new coalition agreement with the Lib Dems and, if necessary, a looser arrangement with the DUP. Nick Clegg has made it clearer than ever in recent days that he is prepared for his party to remain in power with the Tories in return for agreement over his six "red lines": an £8bn increase in the NHS budget, a £2.5bn increase in education funding, guaranteed public sector pay rises, the elimination of the structural deficit by 2017-18, the rejection of £12bn of welfare cuts (as proposed by the Tories) and the protection of the environment through new green laws.

Two of these - an £8bn increase in the NHS budget and the elimination of the structural deficit by 2017-18 - are included in the Conservative manifesto and the remainder, with the exception of the £12bn welfare cuts, could be easily negotiated. Clegg has signalled that he is prepared to accept the Tories' EU referendum pledge, Cameron's greatest red line (he has pledged to resign as PM if he cannot deliver it), in return for concessions elsewhere. 

If the Lib Dems and the Tories are unable to reach 323 themselves, Cameron will look to support from the DUP, who are expected to win nine seats (making them the fifth-largest party after the SNP and the Lib Dems). The Northern Irish party's demands include the maintenance of defence spending of at least 2 per cent of GDP (the Nato target Cameron has refused to commit to) and the abolition of the bedroom tax. 

In extremis, the Tories may seek the backing of the one-five Ukip MPs expected to be elected. But this would make it far harder to win over the Lib Dems, who have pledged not to enter into any arrangement that includes Farage's party. 

3. Labour are the largest party and can form a majority with the SNP and/or the Lib Dems

If Labour win the most seats and can reach 323 with the aid of the SNP (who will become the third-largest party) and/or the Lib Dems, most Conservatives expect Cameron to resign. The Scottish nationalists have vowed to vote down a Conservative Queen's Speech meaning he would face certain defeat under these numbers. 

By far the best outcome for Labour would be to achieve a majority with Lib Dem support alone. That would avoid the need for any association with a party whose raison d'être is the break-up of UK. But the current arithmetic suggests that the two parties would fall short of 323 even if they exceeded expectations. Miliband would almost certainly require the backing of the 50-plus SNP MPs likely to enter the Commons. Having ruled out a confidence and supply arrangement with the nationalists as well as a coalition, the Labour leader would seek to run a minority adminstration and operate on a vote-by-vote basis with the nationalists and others. It would, as I wrote in last week's magazine, require immense political dexterity from Miliband - but the alternatives described above are far worse. 

George Eaton is deputy editor of the New Statesman.