The Unite leader's speech may have been couched in terms of co-operating with Ed Miliband to reform Labour's link with the unions.
But you don't have to look too closely between the lines to see his real message.
The Iraq war, pandering to the City, failing to overturn the Tory anti-union laws - McCluskey's litany of Labour's failures is aimed squarely at the Tony Blair cultists who continue to infest the party's inner circles.
It's a direct challenge to the Blairites who have been waiting years for this opportunity to break the union link and complete Labour's transformation into a third neoliberal party.
And it leaves Miliband himself in no doubt that the battle for the heart and soul of the Labour Party is very much alive.
The Labour leader now has a straightforward choice. Align himself with the tiny clique who did their best to destroy Labour and cost it the votes of millions of once-staunch supporters - dismal, discredited failures like Lord Mandelson, Lord Adonis, Lord Sainsbury and Blair himself.
Or reject them and their fat-cat-friendly policies, give the elbow to the City gamblers that Blair and co courted, and stand alongside the vast majority of people who are under the cosh thanks to the Con-Dem cuts.
Labour's only winning strategy is to embrace the unions that founded it. To accept that Blair's flirtation with the City has done it immeasurable harm.
To be proud of the union link as a way for the party's policy to be shaped by millions of working men and women - rather than a cabal of millionaires and Oxbridge policy wonks.
McCluskey knows that. Miliband needs to realise it too if he wants to go down as anything more than a footnote in his party's history.
By spinelessly letting the right-wing press set the agenda, Ed Miliband is missing a golden opportunity to expose the corruption at the heart of the Tory Party.
David Cameron can bleat all he likes about union influence but the fact remains that trade union funding is the cleanest money in politics.
Meanwhile his own party has some serious questions to answer about the involvement of notorious tobacco lobbyist Lynton Crosby.
The situation remains murky regarding Crosby's possible influence on the government's decision to scrap plans for plain cigarette packaging. And without proof, Cameron and Crosby's denials aren't worth the fag packet they're scribbled on.
This is just the latest incident in a long link between the Tories and Big Tobacco going back at least as far as Ken Clarke in the 1980s.
And that link is just one way in which the Tories are rotten to the core - bankrolled by hedge funds, City banks, non-domiciled tax dodgers and the same privateers who are making a killing from the great carve-up of our public services.
Miliband has enough ammunition there to tear the Tories to shreds - if he can refrain from turning his fire on his own side.