We sometimes forget the power of this link. And given the parlous state of the party's finances, Labour's reliance on their brothers in the unions may become stronger as the next election approaches. It is not so easy to raise cash from major donors, if they think they may suffer a dawn raid from Mr Plod before the ink on their cheque is dry.
So Ed Balls' speech last week to London Business School was more than usually significant. After a lengthy, but interesting, apologia for Labour's policies on investment, competition and corporate governance he moved into a moving tribute to the private equity boys. To listen to Ed, they are a kind of combination of McKinsey and Oxfam, turning companies round, putting them back on their feet, and spreading wealth and happiness throughout the land.
Then came the sting, a treasury review of the tax treatment of 'shareholder debt'. Goodness know what the outcome will be, except fat fees for the accountants. But with one bound Balls put the government back on the front foot.
Are we looking at the next Chancellor? Seems to be between him and Alistair Darling. The answer will depend on whether Brown wants a Chancellor with a mind of his own. Though Balls was born of one of Gordon's ribs, he has his own ideas. So if Brown wants to retain day-to-day control of Number 11 when he moves next door, it will be Darling. If he is prepared for an activist Treasury under new management, it may be Balls.