The Labour Party is currently the biggest in the country, but the size of its, generally Corbynite, membership is now under threat from the MPs' ongoing coup. When Jeremy Corbyn entered the Labour leadership race last year, there was a huge surge in the party's membership which continued unabated for six consecutive months. Many came directly from other parties - 70,000 during the contest itself, including registered supporters - and these are almost certain to resign their membership if Corbyn is ousted, as will many others.
How many members could leave? The most certain to go are those who have joined the party since he was elected leader, often by converting 'registered supporter' status into full membership. No very recent figures exist, but between 12 September and 24 December last year, the net increase in membership was 78,591, or 763 per day, and by January 2016 the total number of members was 380,000, an increase of around 110,000 from August 2015. An overwhelming majority of these members will choose to leave in a very short space of time, many immediately, if Corbyn is removed - perhaps 90,000.
Of the roughly 100,000 members who joined the party during the leadership contest, around 60% voted for Corbyn. Many of these will also have joined the party for the sake of supporting him alone and will leave if he does, perhaps 40,000, likely over the course of a few months.
The least likely to leave the party are those who joined during or before the tenure of Ed Miliband; although a plurality of each voted for him, 49% and 44% respectively, both have demonstrated their capacity to remain in a more moderate party. Nonetheless, many will be disgusted by the anti-democratic opportunism of the MPs and may regard it as the last straw.
It is worth noting that the impact of a Corbyn decapitation could be diminished by the strategic decisions of the Left of the party, which still hopes to retain the leadership. Richard Burgon MP, a committed Corbynite, is currently advising left-wing members that the coup is a ploy to make them resign in protest so that the Right can take advantage of their absence to regain control. However, given the likelihood that the Parliamentary Labour Party will use its right to select the names that will appear on the ballot to exclude left-wing candidates, if Corbyn is toppled then the Left is definitively out of power, and there will be little reason for left-wing members to stay.
What will be the effects of an exodus, which is likely to be in the region of 140,000, by these numbers? Firstly, it could be fatal for Labour's campaigning. The party would lose the organising power of Momentum, which can purportedly mobilise 100,000 volunteers. While many MPs have turned their noses up at the group, it proved its effectiveness in the Oldham by-election, for which it brought in 150 activists. The seat went to Labour on a 7.3% swing. Momentum has also put voter registration at the top of its agenda; since the unregistered are natural Labour supporters, being disproportionately young and poor, the party will suffer if Momentum is no longer reaching out to them.
Then there are the funds. The standard membership rate is £3.92 per month. If this is used as an average rate paid by the resigning members (students and others can pay less but adults can choose to pay more), then Labour stands to lose almost £6.6million over the from a members' exodus over the course of a year. This could be compounded by the withdrawal of donations from left-wing unions such as the Bakers, Food and Allied Workers' Union, which in the last three years alone has given Labour £35,870. In 2014, the party's net income was £4,237,000.
At the same time, with Corbyn gone, corporate donations are likely to flow back into the party. There is already interest among former private donors in excising the current leadership from the party: according to Private Eye, one long-time donor, Sir David Garrard, is currently giving large sums of money to any Labour MP willing to criticise Corbyn. Presumably, with his foe ousted and the party shifted back towards the right, he would recommence pouring the contents of his voluminous pockets into the party. It is up to Labour to decide how it prefers to be funded.