The contrast between the Conservative and Labour leadership contests has already been marked. The Conservatives finished theirs in a matter of days while Labour's looks set to continue until late September. But Labour's problems go a lot deeper than the transient tumult of the leadership election.
The most likely outcome of the Labour leadership contest is that Jeremy Corbyn will be re-elected leader. A YouGov poll of Labour members for the Times in mid-July  has him winning 56−34 over the (then hypothetical) challenger Owen Smith. Even allowing for polling error, that points to a clear victory.
What happens next depends on the Labour members of parliament. Three-quarters of them, 172 out of 232, have already voted against Corbyn in a vote of no confidence at the end of June. They will not welcome his re-election, but what will they do about it? They have two main options: stay or go. But both have strong drawbacks.
If they stay they have to contend with Jeremy Corbyn as leader, as well as the strength of the party's energised left wing, a possible general election defeat, and the potential threat of deselection. And selection processes will really happen since the new boundaries scheduled for 2018 will shrink the number of seats from 650 to 600. Only about one in eight Labour-leaning seats might be left unchanged, and so most seats will face some change to their boundaries. And that means 232 MPs will be fighting for about 200 winnable new seats.
Their hope in staying would be to ride out the deselection risks, anticipate a defeat at the next general election and attempt to regain control of the party and rebuild afterwards. The historical parallel would be the recovery of Labour after Michael Foot's defeat in 1983.
The alternative to staying is leaving. A group of MPs could choose to leave the formal structures of the Labour party, which is equivalent to leaving the party and setting up a new one. One quick advantage would be that the new party could instantly become the official opposition if at least 117 MPs make the change. Their new leader would become the Leader of the Opposition and face Theresa May at Prime Minister's Questions each week. But the rump Labour party would retain the name, offices and (possibly) the annual £6 million of parliamentary "Short" money .
The new party would need a new name and aim to take a good chunk of the current Labour membership with it. But a majority of Labour members would likely stick with Corbyn, since they would have just voted for him. Also Labour enjoys long-term support in much of the country, sometimes from generation to generation. These long-term ties may not be easily broken, and some of those loyal party supporters will stick with Labour through thick and thin. So the new party should aim both to take voters from "old Labour" as well as other existing parties. The obvious historical parallel is the formation of the SDP in 1981 when twenty-eight Labour MPs left the party to create the new Social Democratic Party. The SDP achieved poll ratings of up to 50%, but never won more than six seats at general elections. Could a larger breakaway group do better?
Electoral Calculus has analysed a set of likely scenarios to find out. There is not yet polling evidence on the two key questions: how much would left-of-centre total support change, and what share of it would the new party get. So a range of possibilities is considered. Total Labour support ('Old Labour 'plus 'New Labour') could go up or down, depending on how messy the separation looks and the electoral appeal of the new grouping. The scenarios run from −5pc (total Labour support decreases from 31pc to 26pc) up to +20pc (support increases to 51pc). The split of these votes between 'Old' and 'New' is perhaps more likely to favour the new grouping, particularly if they can gather a majority of existing MPs. These scenarios run from 50pc (total Labour votes split equally between the two successor parties) to 70pc ('New Labour' gets 7 Labour votes out of every 10).
|Overall Labour vote share change on 2015|
|'NewLab' share||Party||Lab −5%||Lab +0%||Lab +5%||Lab +10%||Lab +15%||Lab +20%|
|50 / 50||Con||407||394||381||370||358||344|
|60 / 40||Con||403||390||377||366||351||337|
|70 / 30||Con||394||379||366||354||339||319|
©2016 Electoral Calculus
Table: Seats won by the major parties if Labour splits under various scenarios (using old seat boundaries)
The table has a stark warning for Labour. In almost all the scenarios, the combined Labour parties win fewer seat than they did at the last general election. Only if overall Labour support increases by 20pc, do they equal their 2015 performance. And in all cases, the Conservatives remain the largest party in parliament and are almost certainly in government.
Let's look at a central realistic scenario where overall Labour vote goes up 10pc and 'New Labour' takes 60pc of the split. Then the Conservatives win 366 seats to have an increased majority of 82 seats, 'New Labour' is on 141 seats and 'Old Labour' has only 59 seats.
And this is before the effect of the new constituency boundaries. Using the projected new seat boundaries the Conservatives win 351 seats to have a majority of 102, with 'New Labour' on 125 seats and 'Old Labour' on 52 seats.
In a less favourable scenario, where opinion polls stay at current levels, then the results with the projected new boundaries are: Con 391 seats (182 seat majority), 'New Lab' 92 seats, 'Old Lab' 38 seats (well behind the Scottish Nationalists).
The message to Labour MPs is clear: look at the fire before leaping out of the frying pan.
-  YouGov, Survey of Labour Party Members, YouGov / The Times (19 July 2016). URL https://d25d2506sfb94s.cloudfront.net/cumulus_uploads/document/w0mr4c6hq9/TimesResults_160718_LabourMembers_Website.pdf
-  Meg Russell, "What if Labour splits?", Constitution Unit (29 June 2016). URL https://constitution-unit.com/2016/06/29/what-if-labour-splits/