Liz Kendall has demonstrated the extent to which Labour is in hock to the trade unions by announcing her first policy - to reverse the Conservative Government's proposed laws to weaken the power of trade unions.
She is the leading Blairite candidate in the race to replace Ed Miliband as leader and therefore the least trade union-friendly figure in the contest.
But by writing an open letter to union members today - pledging to "strengthen your relationship with the Labour party" - she has only shown her party as one that is still bound by the diktats of the unions.
Ms Kendall's concerted effort to woo union members came on top of new figures published today that revealed the financial power the unions also wield over the party.
A staggering 71 per cent of Labour's donations during the election campaign came from trade unions. A quarter of those funds - £1.6 million - were donated by Mr McCluskey's Unite trade union.
It takes the full amount donated by his union to £15.9 million since 2010.
After the unions were credited with sweeping Ed Miliband to power in the 2010 Labour leadership - with the one third trade union vote going towards the younger Miliband brother - the new Labour leader introduced reforms to the election rules in order to weaken their influence over the decision.
Who will be the next Labour leader?
Who will be the next Labour leader?
1/7 Andy Burnham
Andy Burnham has promised to restore the party's "emotional connection with millions of people," if elected
2/7 Mary Creagh
Mary Creagh has called on her party to win back “Middle England”
3/7 Liz Kendall
Shadow health minister Liz Kendall is seen as a Blairite
4/7 Yvette Cooper
Shadow home secretary Yvette Cooper became the fourth person to join the Labour leadership race
5/7 Tristram Hunt
Tristram Hunt, the shadow Education Secretary, has said he will not run for the Labour leadership as he had not gathered the required nominations of 35 MPs. He has instead endorsed the moderniser Liz Kendall.
6/7 Dan Jarvis
One of the favourites to succeed Ed Miliband as Labour leader – ex-Army paratrooper Dan Jarvis – has ruled himself out, saying he won't do it because of his children
7/7 Chuka Umunna
Chuka Umunna dropped out of the Labour leadership contest just three days after he announced he was in the running
But it does not appear to have weakened them as much as the Blairites in the party had hoped. Ms Kendall, referring to David Cameron's manifesto pledge to crack down on the ability of trade unions to call strike action by introducing minimum turnout thresholds in ballots, said it would be a priority of any government she led to reverse them.
"I will tolerate no weakening of protections for working people or the basic rights of trade unions while I’m leader," she wrote in an open letter to all union members on the New Statesman website. "If they’re implemented by this Tory government, the Labour government I will lead will reverse them."
A look at the 631 candidates Labour put up at the election also demonstrated how solid and infiltrated the union power-base is within the party.
That explains why modernising leadership candidates such as Ms Kendall and Mary Creagh could struggle to find the 35 Labour MPs they need to nominate them to enter the race.
The Independent revealed fears among allies of Ed Miliband that Mr McCluskey's Unite union was putting pressure on Labour MPs not to nominate the Blairites in order to limit the race to the two candidates favoured by the unions - Andy Burnham and Yvette Cooper.
There were also reports that the same union was using a team of cold-callers to influence the outcome of the race to become the Labour candidate for London Mayor.
One Labour councillor alleged he was "subtly" encouraged to back Sadiq Khan, the former shadow justice secretary, by a Unite cold caller who described Mr Khan as a "great guy" and emphasised his background as a son of a working class bus driver.
Meanwhile Mr McCluskey appeared to threaten his union's future financial backing of the Labour party earlier this week, saying he could sever Unite's links if the party failed to choose the "correct leader" and pointed to the many Scottish members of his union that supported the SNP. This would do serious damage to Labour's 2020 election war chest.
Conservative MP Michael Ellis said the figures proved that Mr McCluskey was the "most important figure in the Labour party".
"He buys the policies and picks the leader," he said. "It’s sad seeing people like Liz Kendall go begging to the union bosses for support. It’s the same old Labour - run for the benefit of trade unions, not for working people."
A Labour party spokesperson said:
“Labour party members and small donors give us the largest proportion of all our donations. We welcome support from millions of nurses, cleaners, cooks, shopworkers, paramedics, teaching assistants and building workers who are members of trade unions, unlike the Conservatives who increasingly rely on the money of hedge funds, and an exclusive group of super rich donors – the same people who they have rewarded with tax breaks.”