The GMB union is to marshal its 610,000 members and "substantial" sums of money to campaign against a change to the voting system.
In a move that will put the union at odds with Ed Miliband, its preferred candidate for Labour leader, the union's general secretary, Paul Kenny, will play an active role in opposing the alternative vote in the referendum. The campaign, which will target members and the wider public, is expected to be backed by a six-figure sum from union funds.
The revelation is the first sign of campaigners on both sides of the debate preparing for a multimillion-pound marketing drive, with grass-roots movements claiming huge amounts of money will be raised by rallying activists through Facebook and Twitter.
Under Electoral Commission rules, a "designated organisation" will officially front the "yes" and "no" campaigns, and will be limited to spending £5m each. They will be allowed TV broadcasts, free leaflet delivery to voters, free use of public rooms for meetings and up to £600,000 of public money. With only nine months before the Government wants to hold the vote, a fundraising race is already under way.
The GMB's policy to retain first past the post creates an unlikely alliance with the main "no" campaigners, drawn mainly from the right of the Conservative Party. Tory peer Lord Leach of Fairford and backbench MP George Eustice, veterans of the campaign against the euro, are fronting the opposition to AV. Mr Eustice said yesterday: "Our one-person, one-vote system is simple and gives us strong government. AV does nothing to help smaller parties like the Greens and it means that some people get two votes counted while others only get one."
However, Mr Kenny stressed that the GMB – which gave £1.4m to Labour last year – would not be forming an alliance across the political divide. "We are not going to get into bed with everybody who opposes a change in the voting system," Mr Kenny said.
It is understood that the "yes" campaign is preparing to appoint a high-profile campaigner to push for AV, stressing the need to explain that ranking election candidates in order of preference will produce a "fairer" outcome. Ashley Dé from the Electoral Reform Society said the "yes" campaign faced "an uphill struggle" to match the "high-rolling donations" which the "no" campaign is expected to attract. "We are going to need every penny. It is not expensive to administer, but we need money to win. We are talking in the millions," he said.
Andy May, the national co-ordinator of Take Back Parliament, is building an activist base of thousands. "We will be genuinely grass roots and cross-party, rather than a narrow tranche of the right of the Conservative Party."
For voters, the prospect of unions and Tories uniting against reform will complicate an already murky picture of who stands where. The Liberal Democrats will back AV, despite Nick Clegg describing it last year as a "timid" reform, while Labour opposes the legislation because the coalition also wants to redraw constituency boundaries. The Green Party leader, Caroline Lucas, said AV was a "small step" in the right direction but "won't transform politics, and it won't open up the House of Commons to diverse voices".
More than 40 Tory MPs could derail the progress of the Bill when it reaches the Commons in September, because the proposed referendum date of 5 May next year coincides with national elections in Wales and Scotland, which they claim will skew the result. Bernard Jenkin, the Tory MP who tabled a Commons motion calling for a rethink, said: "There is a great issue of principle at stake – either having a fair referendum or a fiddle."
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