Living standards may be falling and Labour's poll lead widening on the back of Ed Miliband's pledge to freeze energy prices. But to judge by the outpouring of poison from David Cameron and his media friends, the Conservatives have finally come up with a strategy to win the next election. Their plan can be summed up in one word: Unite.

For the past few months, the prime minister has lost no opportunity to denounce the country's largest union (and Labour's biggest affiliate) in the most lurid terms. He has poured vitriol over its general secretary, Len McCluskey, portraying the former docker as a scouse Svengali and Miliband's puppet-master. A fortnight ago he went lower still, branding Unite's sacked convenor at Grangemouth, the respected Stevie Deans, a "rogue trade unionist who nearly brought the Scottish petrochemical industry to its knees".

In one Prime Minister's Questions, Cameron mentioned Unite and McCluskey eight times. But this is just the warm-up. Egged on by his election strategist, Lynton Crosby, Cameron – government sources say – plans to ratchet up the campaignagainst Unite still further. It's the ideal device, Tories reckon, to paint Labour's leader as a weak 1970s throwback.

Cameron's press outriders have been preparing the ground for weeks. Outraged that Miliband dropped Labour's suspension of Deans and the Unite-backed candidate, Karie Murphy, in the Falkirk parliamentary selection after claims of rigging fell apart, the Murdoch press and the Mail group have declared open season on the union.

The Sunday Times and Daily Mail have led the field, splashing stories of supposed Unite skullduggery week after week. Murdoch's weekend flagship has tried to revive the Falkirk row, using emails culled from Deans's Grangemouth computer, and cast spurious doubt on McCluskey's own re-election this year – with 144,570 votes, more than 60,000 ahead of his rival.

The Mail has widened the field of attack to Unite's industrial tactics, slipping into full 1980s revival mode. Under the headline "Terrorised by union bullies", the paper claimed the union was running a "sinister" campaign of "intimidation" and "dirty tricks", after Unite activists staged a silent protest with a giant inflatable rat outside the homes of Ineos executives.

The media onslaught has dovetailed with the corporate attack on the union at Grangemouth. Far from the union closing down the plant, it was the employer that locked out the workforce last month until they accepted worse terms and conditions. Unite had threatened to strike after Ineos used the Falkirk controversy to suspend Deans. The Ineos owner, Jim Ratcliffe, has now made clear that he repeatedly put off confrontation with Unite until the company was ready.

Just as striking is the fact that emails from Deans's computer were given to the Sunday Times the day after his disciplinary hearing and effective dismissal three weeks ago. An Ineos spokesman told me yesterday that the company had passed them to the police and information commissioner, but would neither confirm nor deny if it had also arranged for the emails to be given to the press.

Either way, a company about to benefit from a £125m government loan guarantee has certainly given a helpful boost to Cameron's campaign. The emails confirm what we knew already: that Unite recruited a large number of industrial workers to a moribund local Labour party and supported the progressive Murphy to be Labour's candidate.

Beyond that, the partial and potentially libellous internal Labour report that triggered Miliband's intervention and call to reform Labour's union link has been shown to be at best misleading. Its main allegation, that a handful of people were signed up without their knowledge by other family members, has been superseded by new evidence. Affidavits from members of the Kane family, at the centre of the dispute, have been passed to the Guardian. They confirm that none had a problem about joining, none was a union member or paid for by Unite, and one was actually planning to vote for Murphy's opponent.

What were in fact the kind of cock-ups and local disputes that are, as Labour's report puts it, "not unusual" in such contests have been transformed into an ugly smear campaign – used to break the resistance of a well-organised industrial workforce and now being relentlessly exploited to try to derail Miliband's growing political momentum. The aim is clear enough: to portray the Labour leader as hopelessly in hock to trade unions (which in reality have remarkably little influence on Labour policy), while diverting attention from falling living standards – and the commanding grip of the far more powerful City and corporate interests on the funding of the Conservative party.

But the onslaught on Unite isn't only about Labour, let alone the storm in a Falkirk teacup. McCluskey's union, the strongest in the private sector, has shown that its combative tactics have delivered results. Its "leverage strategy" – what the Mail calls terrorism – aims to offset industrial weakness by targeting the suppliers, customers and shareholders of recalcitrant employers with lobbying and demonstrations. To their dismay, it has proved effective: from Honda and London buses to construction and Crossrail.

Not only that, but Unite has gone out on a limb to demand the collective working class political voice squeezed out of the Westminster and media magic circle for a generation – which is what lies behind Falkirk. And, with other unions, it is one of the only organisations with real social weight and resources that opposes an entrenched neoliberal consensus and corporate elite.

So it's no wonder those vested interests that benefit want to cut Unite down to size. The Sunday Times worries about the "shift to the left" in public opinion it fears Miliband is tapping into. The only way the Labour leader can head off Cameron's Unite propaganda war is to get off the back foot and turn the tables on the City and big business lobbyists that stand behind the prime minister and his party.

In any case, there's little sign that Cameron and Crosby's Unite baiting is going to work. It's a retro 1980s strategy that doesn't chime with people's experience of Britain in 2013: falling real wages, shrinking public services, insecure jobs and corporate profiteering, not overweening unions. But it won't be seen off if Unite is left to fight alone.

Twitter: @seumasmilne