Think back to 2010, when Diane Abbott's presence on the ballot helped elect Ed Miliband

Much is made of Jeremy Corbyn’s presence in the Labour leadership race, and his potential impact on the debates. Rather less is being made of Corbyn’s impact on the electoral arithmetic.

Thanks to the ‘Alternative Vote’ (AV) system, the abolition of the Electoral College, and the new ‘registered supporter’ rules, Corbyn could change the electoral maths every bit as much as he changes the debate.

Use of an AV system undoubtedly changed the outcome of the 2010 Labour Leadership race.

David Miliband was initially 3.5 per cent ahead on ‘1st preference’ across the Electoral College.

Using a First Past the Post system, he would have won and become Labour leader. It was only after the 2nd preferences of those who had voted for Abbott, Balls, and Burnham were tallied up, that Ed came through — by just 0.5 per cent.

For the 2015 contest, the Electoral College has gone but the ‘Alternative Vote’ (AV) remains. Voters will rank their candidates in order of preference. Each voter can list as many or as few candidates as they please.

Candidates need more than 50 per cent of the vote to win. If the leading candidate does not get 50 per cent of 1st preferences first time round (as David Miliband didn’t), the candidate with the fewest 1st preferences drops out and their 2nd preferences are then tallied up.

And so on and so on, till a candidate gets over the 50 per cent mark.

According to Stephen Bush, the well-informed editor of the Staggers blog, there has developed something of a consensus amongst some Labour Party members that the presence of Diane Abbott, a black woman of the old-fashioned left, helped – via the AV system — push Ed through in 2010.

The theory is this: significant numbers of lefties / Diane Abbott fans, who without Abbott on the ballot would not have voted at all, voted in the race. Let’s call these voters Because Abbott is on the Ballot Voters (BABVs).

When BABVs turned out, they tended to put Ed Miliband — the non-‘Blairite’, non-Iraq war candidate — as their 2nd preference on the ballot and, as a consequence, Ed was sufficiently helped and David was sufficiently hindered to account for Ed’s margin of victory.

From the 2010 Summary of Voting by Round, we see that Ed gained 2 per cent on David overall when Abbott dropped out of the contest.

Looking just at the members’ section of the Electoral College, the figures show that among those Party members who put Abbott first, three and half times as many put Ed second as put David second.

To uphold the Stephen Bush theory, two assumptions are necessary.

1. Abbot’s 2nd preferences were (roughly) evenly distributed between BABVs and those who would have voted whether or not Abbott was in the contest. 2. Around two in three of Abbott’s 1st preference voters were BABVs.

If these assumptions are both correct, we can confidently say that Abbott’s name on the ballot would in effect have been (more or less) responsible for the entirety of the 0.5 per cent margin that Ed won by.

Both assumptions seem fairly plausible, but of course we can’t make them with any certainty.

BABVs may have contributed to the margin of victory but not to the victory itself. We simply don’t know how many BABVS there were. But we can confidently say that as long as there were significant numbers of BABVs voting – and it has been reported that there were — they helped Ed and they cost David.

That was 2010. What about 2015? Could Corbyn change the arithmetic in the same way as Abbott did in 2010?

Yes. Indeed he could have much more of an influence.

As Abbott was in 2010, Corbyn is the only solidly socialist candidate and, with the Electoral College gone, each member’s vote is effectively worth more than it was in 2010.

It is also much easier and cheaper to vote in the contest than it was in 2010. Any ‘registered supporter’ can now vote for a £3 fee. Who knows who might pay the fee?

Conservative supporting writer Toby Young has openly urged his readers to derail the Labour Party by becoming ‘supporters’ and voting for Corbyn (#ToriesforCorbyn).

He paid the £3 fee himself and he registered to vote. Since then, the Labour Party has tried to ‘weed out’ these mischievous ‘Tories for Corbyn’.

In registering as a supporter now, you not only represent yourself as supporting the Party’s aims and values, but must also sign up to the statement ‘ I am not a supporter of any organisation opposed to [the Labour Party].’

Much more likely, though, to make a significant difference to the end result, is that lefties and anti-austerity protesters etc., who would not otherwise have voted, will register and vote for Corbyn in fairly large numbers.

Let’s call all these voters BCBVs: Because Corbyn is on the Ballot Voters.

Corbyn, at 16 to 1 to win the race, may well still finish last on 1st preferences. But the 2010 example shows that this doesn’t mean he won’t alter the arithmetic. Very likely the 2nd preferences of the BCBVs will matter.

In the 2010 race, 94 per cent of Labour Party Member voters whose 1st preference was Diane Abbott put a 2nd preference on their ballot paper. It seems safe to assume that significant numbers of BCBVs will assume a 2nd preference too.

Of course some won’t. Having entered the race on account of Corbyn, they might put Corbyn as first preference and leave it at that. But once a voter has a ballot and a list of candidates in front of them, the temptation is surely to put a ‘2’ against a 2nd name.

It’s very little effort, and who knows? You could stop your least favourite candidate from leading the Labour Party. Liz Kendall, whom many BCBVs will brand a ‘Tory’, is likely to be least favourite among BCBVs.

The conventional wisdom is that Cooper, whose politics sits somewhere between Kendall and Burnham, is most likely to benefit most from the AV system. While she may not win on 1st preferences, she may mop up the majority of the 2nd preferences of both Kendall and Burnham, so the argument goes.

She may indeed. But the conventionally wise fail to take account of BCBVs. If Burnham wins in a close contest, then Corbyn’s name on the ballot may well play a part in getting him over the line.

David Miliband famously urged his MP supporters to get Abbott on the ballot, something that he surely came to regret. He wanted a broad debate, he said. But he must rue not having paid more attention to how the arithmetic would play out.

Burnham also urged his MP supporters to get Corbyn on the ballot. He also said he wanted a broad debate. But it could be that unlike David, he has been paying careful attention to the arithmetic, and knows that he will be helped by a more left-wing electorate.

Peter Wiggins is a freelance journalist. Follow him on Twitter

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