There has been a sharp increase in Labour party membership - around 32,000 people have joined the party since May. Estimates of total membership suggest this is between 160,000 and 170,000, though the number of ballots to be issued remains open until September 8th, since anybody who
joins the party before then can vote in the election.
So new members now make up a striking 19% to 20% of the party membership, and so over 6% of the whole electoral college. (Turnout among party members was 69.1% in 1994, but new members may well be slightly more likely to vote, both because some have joined specifically to vote, and more prosaically because a small proportion of long-standing direct debit members would not have updated their current address details. If this happens, it would marginally increase their importance).
So could they decide the election? The answer, in principle, is yes.
In one sense, what Labour's new members think would prove decisive in any very close contest. For example, the final round of the 2007 deputy leadership saw Harriet Harman defeat Alan Johnson by 50.43% versus 49.56% in the electoral college, a similar 0.8% margin of victory to that by which Denis Healey defeated Tony Benn in the 1981 deputy contest. (The three contested Labour leadership elections since 1983 were all won by large margins).
But that is not quite the same as a scenario where the presence of the new members means the outcome is different. That would not happen in a close race where both current members and new members prefer candidate A to candidate B by 51% to 49%.
Here is a scenario in which the new members make all the difference.
Firstly, imagine that small leads for each of the final two candidates in the Parliamentary and affiliate sections were to cancel each other out almost exactly. (For example, as a rough back of the envelope estimate, one candidate might lead by by 20 MPs yet be behind by perhaps something like 40,000 trade union votes out of 500,000 returned ballots)
Secondly, that would make the members' vote decisive. Lets say 89,000 not-new members cast votes in the final round (that would be 70% valid votes in the final round from 127,000 members) which favour candidate A over candidate B by 51-49% in the final ballot.
Candidate A: 45,390
Candidate b: 43,610
Those 1800 votes would give candidate A the leadership - the decisive 2% lead among party members would translate into a 0.66% margin in the electoral college, similar to that in previous knife-edge deputy contests.
But that's before we count the new members votes too in the membership section. Let's say there are 25,000 votes here. (75% turnout out of 33,000 would be 24,750 votes).
If candidate A can split new members 50-50 too, they will still win. But a 60-40 split among new members for candidate B is worth a 5000 vote margin (15,000 - 10,000). A 70-30 split is a 10,000 vote lead, and even a 55-45 split (2500 votes) would be enough in such a close scenario as this, where everything else comes out almost even.
Finally, every campaign has another week in which it can seek to expand the electorate, by persuading supporters who are not members to join the party. There are opportunities as the race receives more media attention this week. There are further opportunities among the 500,000 to 750,000 likely to vote in the affiliate section. If a campaign could persuade supporters in this section who are not party members, persuading them to join the party too before 8th September would shift their voting power from around a fifth or a quarter of that of a party member to 1.25 votes in the contest. This is unlikely to happen on an enormous scale - but a thousand or more new member votes could be very valuable.
So might new members might split differently from existing members? There are reasons to think that they might, though it would be dangerous to overstate these.
A plausible hypothesis is that new members may be more likely to vote for candidates they perceive as representing "change" rather than "continuity". Existing members may be more likely to heed warnings - such as those from Peter Mandelson - about the risks of change from the New Labour script.
Every candidate will appeal to new and current members. Every candidate argues they are the leader most likely to defeat the government and get back to power. If new members have been fired up by the Coalition, then they might particularly warm to Ed Balls' combative opposition to the government, for example. Overall, new members are likely to be as interested in Labour defeating the Tories as the rest of the membership.
Every campaign has been trying to recruit more new members too. One important goal of the David Miliband house parties and movement for change rallies; Andy Burnham was trying to do something similar with his large launch event in the north-west and his battlebus tour. Diane Abbott has been encouraging more people to strengthen the Labour left's voice.
There isn't enough information on who the new members are to provide any definitive verdict of how similar or different there are. The media have been interested in the idea that these are LibDem defectors, which is probably overstated. There probably are a good number of ex-party members in the joiners cohort - either because they left over a specific issue, or became disillusioned, or just felt less politically engaged - along with "never members" who are broadly Labour but stayed out for similar reasons. Examples can be found among new recruits from the blogosphere - such as Sunny Hundal from Liberal Conspiracy, or those like Lisa Ansell or Jim Monaghan who have blogged on their decisions to join, though they would not of course claim to be representative of all new members.
However, the YouGov poll also shows that existing members are also to the left of voters generally, and Labour voters too, perhaps also reflected in views about why Labour lost, so this may not differentiate new and existing members as sharply as some may imply.
And those who were broadly Labour but somewhat disillusioned with the party in power will not be the only joiners. There has long been a very large pool of Labour identifiers and supporters who are not members: the Fabian Society Facing Out research identified a pool of 2.5 million non-members who are politically engaged outside the party and who are Labour identifiers, identifying the need to lower barriers to entry to party politics and to ensure members voices counts . The new members may well have a younger demographic - Harriet Harman has said that 30% are under 30 and 80% had joined online; and one driver of some people deciding to join was seeing or hearing about others doing so, including through online social networks.
How new members will vote is not the most important unknown in this election: the (undeclared) second preferences of MPs and MEPs matter more. With at least 60+ Parliamentary votes up for grabs, they are worth 20-25%+ of the Parliamentary section.
(So another way of looking at the new members' influence is that a 70-30 split among them offsets perhaps 10 MPs, or slightly more).
The second preferences of not-new members may well have a broadly similar weight to the overall votes of new members (on first and later preferences). We could estimate that these could be worth around 7-8% of the electoral college.
If around a third of members vote for candidates who do not make the final round, second preferences overall (combining those of new and old members) would be worth around 10% of the electoral college, with affiliate second preferences worth something similar. This will be higher if the final two candidates get fewer than two-thirds of first preference votes between them (and lower if their first round share is greater).
Clearly, new members won't decide the election against the strong preference of other members. But their voices will have an important weight and influence - and it could be decisive in an evenly balanced contest.
Persuading and inspiring more people to join Labour is one of the remaining ways in which candidates and their supporters can affect the outcome. So if you care about who wins the contest, joining the Labour party could be one way to help to make it happen.