It is normally sensible to check the small print beneath politicians’ rhetorical flourishes but Ed Miliband’s claim today to be introducing the most far-reaching changes to the way Labour selects its candidates since John Smith’s introduction of one member one vote is, if anything, an understatement.

Let’s dispense first with the myth that anyone serious in the Labour party wants to break the link with the trade unions. They don’t. Miliband’s proposals are instead about empowering their rank and file members. ‘The problem,’ as he said, ‘is not that these ordinary working men and women dominate the Labour party – the problem is that they are not properly part of all that we do. They are not members of local parties, they are not in active in our campaigns.’

Miliband’s call for trade unionists to choose to join the Labour party through the affiliation fee is an audacious gamble and a huge opportunity. Yes, there is a risk that many of those who have thus far not chosen to opt-out of the political levy do not decide to engage with the new relationship Miliband is offering them. And this could cost the party millions. But let’s not forget that over 200,000 trade unionists ticked the box to say they supported the Labour party and cast a vote in the 2010 leadership election. Their involvement alone could double the size of the party virtually overnight.

While Miliband has rightly said that these changes have implications for the Labour party and the trade unions which need to be worked through, some of those implications are already clear and the line between those who join the Labour party as levypayers through a trade union and those who do not will become increasingly blurred. The electoral college by which Labour elects its leader will inevitably have to go. In its place one member one vote, with the votes of all party members given the same weight –this will enhance the potential voting power of hundreds of thousands of trade unionists at a stoke. Votes at Labour party conference will also need to be reformed. Currently, the union block votes account for 50 per cent of the vote, with the remaining 50 per cent in the hands of constituency Labour parties. This is unsustainable. Instead, it should be reformed so that the union and CLP share falls to one-third, with parliamentarians, councillors or the party’s National Policy Forum taking the final third.

Trade unionists who pay the political levy don’t currently get a vote in parliamentary selections. The logical conclusion of Miliband’s reforms is that they must, with union and local party branches retaining the right to nominate candidates.

Miliband’s support for a primary to choose Labour’s candidate for the London mayor and his call for an examination of the use of primaries in cases where a Labour MP stands down and the local party has insufficient members to allow a ‘properly representative selection process’ was the other major component of his speech. With strict caps on spending, members retaining the right to short list candidates (anyone who gets a union or party branch nomination should be allowed to stand), and the electorate composed of all those willing to register their support for Labour, primaries could, as Miliband suggested, play a major role in ‘re-energising’ the party. This is a huge victory for those like Tessa Jowell who have waged a sometimes lonely campaign for a more open politics through primaries for a number of years.

The big question now, of course, is: can he deliver? While the reaction of some trade union leaders was predictable, this will not deter Miliband. He will have known full well what he was getting into. As I argued yesterday, to ease the passage of his reforms, he should take up the proposal made by the trade unions through the Trade Union and Labour party Liaison Organisation’s submission to the Refounding Labour party reform consultation to give ‘equal weight to constituencies, trade unions and other stakeholders’ on the party’s National Executive Committee. With parliamentarians members and unions and affiliates each having equal representation on Labour’s key decision-making body, there would be an opportunity for each English region and Scotland and Wales to elect a representative, boosting the number of those elected by party members from six to 11.

And Miliband retains another card to play: as Tony Blair did over clause IV in 1995, Miliband could call a ballot of all party members to vote on his proposals. It is a vote he would resoundingly win – and one his opponents would be ill advised to risk.