Andrew Grice: This referendum is a tricky one for Miliband

The "gay wedding" joint press conference in the Downing Street garden seems a long time ago. Yesterday David Cameron and Nick Clegg appeared on different television programmes to set out their opposing views on the electoral system. Today the Prime Minister and the Liberal Democrat cabinet minister Vince Cable will appear at rival events for the No and Yes camps ahead of the 5 May referendum on whether to switch to the alternative vote (AV).

The third man in the debate, Ed Miliband, makes a significant intervention today in his article in this newspaper. Unlike the Conservatives and Liberal Democrats, his Labour Party is split down the middle on AV, so his position is trickier.

Mr Miliband came under enormous pressure from Labour colleagues to join the No camp to "kick Clegg" for hopping into bed with the Tories. He resisted it. He personally favoured AV long before the Liberal-Conservative coalition was dreamt of; he wants to argue for what he believes is right for the country and he wants to be on the side of the "new politics" because that was a key part of his pitch in Labour's leadership election.

But he decided not to share a Yes platform with Mr Clegg, arguing that the unpopular Liberal Democrat leader should keep his head down to avoid damaging the pro-reform cause. He has a difficult balancing act: he also has to try to maintain Labour unity, so today's attack on "scaremongering" by the No camp is aimed at its Tory, rather than Labour, figures.

Mr Miliband is playing a long game. The voting system should not be decided by short-term politicking or personalities, he believes. People should judge electoral reform on its merits: they shouldn't vote No to spite Mr Clegg or Yes to damage Mr Cameron. Referendums in Ireland and on the Continent about Europe have shown that voters tend to express a view about the government of the day, rather than the issue on the ballot paper. This makes life complicated in a coalition of two parties on opposing sides. It means that Labour supporters could easily decide the AV referendum.

Privately, Labour backers of AV fear that the desire among the party's supporters to punish Mr Clegg may trump Mr Miliband's plea – especially in the North, where Labour and the Liberal Democrats go head-to-head in council elections on the same day.

They send out a slightly different message on the Labour grapevine: punish Mr Clegg in the council polls but hurt Mr Cameron and George Osborne by voting Yes in the referendum. That would be a win-win for Mr Miliband rather than a double blow for Mr Clegg. It would damage the leaders of both coalition parties, not just the junior partner. Mr Cameron, having granted the referendum in the post-election coalition talks, would have a lot of explaining to do to his restive backbenchers. A Yes vote for him could be harder to explain to his party than Mr Clegg's task in selling a No verdict to his.

"A No vote cast to spite Nick Clegg for forming the Coalition is a vote that bolsters George Osborne, the cuts and his strategy for a Tory majority in 2015 on the existing voting system," said Sunder Katwala, general secretary of the Labour-affiliated Fabian Society. "The one result that would really rock the Coalition would be big Liberal Democrat losses in the local elections combined with a Yes vote on AV. That would see Mr Cameron facing a furious revolt from his backbenchers and grassroots Tories, as the man who failed to win a majority last May and who had now lost the electoral system they love too."

Mr Clegg's allies suspect Mr Miliband is playing a double game. They regard his "don't kick Clegg" plea as his insurance policy: if the public votes No, the Labour leader could argue that he tried his best but was handicapped by Mr Clegg's deep unpopularity. Liberal Democrats believe Mr Miliband could, and should, have done more to rally Labour troops behind the Yes banner. As nobody knows the outcome on 5 May, the blame game may have already begun.