Will Len McCluskey ever shut up? His call for Labour to rule out a coalition with the Lib Dems after the 2015 election (as reported in last Sunday’s Observer), or else face cuts in funding from the Unite union he leads, is as unhelpful an intervention as it gets. It fuels the myth that Labour is controlled by union barons and makes another term of Tory-led Government beyond 2015 more, not less, likely.

The only good news is that the ongoing turmoil of the Scottish referendum campaign meant that the TUC congress, and the crass stupidity of Len McCluskey’s comments, did not get as much media coverage as they would have normally got. I hope Ed Miliband, and the TUC, ignore him.

I agree that the Lib Dems have committed many errors, and betrayals, in office – but they had little choice but to join the Tories in a coalition in 2010. A rainbow coalition of Labour, the Lib Dems, SDLP, Green, Scots and Welsh Nationalists  was never on the cards, whatever Andrew Adonis may say. Even if it had been cobbled together it would not have survived long: given the increasingly venomous Scottish referendum campaign, can you imagine a coalition of the Lib Dems, Labour and Alex Salmond’s SNP still in office today? And imagine the opprobrium that would have been poured on the Lib Dems for having teamed up with a defeated Labour party that had only gained a pitiful 29% of the popular vote.

Four years on, bashing the Lib Dems is still an amusing bloodsport for some on the left but it gets us nowhere nearer the outcome we should all be working for: a Labour (or Labour-led) government that reverses Tory policies and stands up for social mobility, equality and public services. The Campaign for a Labour Majority is all well and good, but winning outright in 2015 after Labour’s second-worst ever result in 2010 is a tall order and we need to make contingency plans in case we don’t. Ruling out a coalition with the Lib Dems at this stage, just because they got in bed with the Tories in 2010, would only make another disastrous spell in opposition more likely. And as we saw with last week’s vote on the Bedroom Tax, the Lib Dems are already people that Labour can do business with. It’s time for Labour to move on from the “betrayal” of SDP defectors in the 1980s, and the perfidy of the Lib Dems in 2010.

Why continue to attack the Lib Dems so virulently? In Greenwich, where I was a councillor until recently,  the Lib Dems always got a lot of stick from Labour even as their councillors fell from four to two in 2010, and then from two to zero in 2014. It all seemed rather pointless at first, but it became clear that bashing the Lib Dems is, for Labour politicians, a bit like talking about the weather: it fills the time when you have nothing else to say. What’s more, it can be a useful way of distracting attention from the weaknesses of your own argument. And  it can always be relied on to get a cheer from your supporters at the Trades Union Congress, the Town Hall or in Westminster.

Out in the real world, attacking the Lib Dems strikes undecided voters as yet more bullying tribalism, like beating a man who’s down. Such sadism brings no satisfaction and is likely to drive wavering  Lib Dems into the arms of the Greens – or to stay at home. It’s often glibly assumed that most Lib Dem voters in 2010 will automatically drift to Labour in 2015. But polling show that only about a third say they definitely will (see Leo Barasi’s analysis here): the rest are still up for grabs.

The persuasion of Lib Dem voters to vote Labour in 2015 is work in progress, and is hardly helped by Len McCluskey ordering Labour not to even consider a deal with their last party of choice. Even Iain Dale – a Tory blogger who is no friend of the Lib Dems – has predicted that thanks to the personal vote many Lib Dem MPs have built up, 30 to 35 of them could survive even if their national vote share stays at around 10%. What’s more, even if Labour wins the popular vote it’s far from certain that Labour will win a majority of seats, resulting in another hung Parliament.

Polling shows that the trade unions are, like it or not, a handicap for Labour in the run-up to the 2015 election and that, like it or not, the Lib Dems will still be a force to reckon with after it. That’s not to say that trade union leaders should be silent in the meantime – far from it. But I wish they would stick to standing up for their members and highlighting the effects of coalition policies, not feeding the Tories’ narrative that the unions call all the Labour Party’s shots.

A Lib Dem-Labour coalition could in  fact be one of the more desirable options available after the 2015 election – compared to, say, a renewed Lib Dem-Tory coalition or even a UKIP-Tory coalition, which might just happen. To threaten Labour with a cut in union funding if a coalition with the Lib Dems goes ahead is not just wrong. It is also a daft thing for Len McCluskey to say publicly, even if he ponders it in private.

There’s no mileage – and a lot of danger –  in bashing the Lib Dems yet again. And as I argued in a blog post back in July, Labour needs to show, not tell: Labour shouldn’t have to explain why we are a better party of Government than the Lib Dems. If that isn’t obvious by now we don’t deserve to win the 2015 election at all.