Ed Miliband's proposal to open Labour leadership elections to non-members has all the makings of a "Clause Four moment", a counterpart to Tony Blair's historic constitutional shake-up of the party in 1995. What remains to be seen is whether he can pull it off.

Mr Miliband casts the plan as a response to the demise of card-carrying political tribalism, a way to tap into Labour's increasingly inchoate support base. His analysis of modern political sensibilities is accurate enough. It is also a fig-leaf for a direct challenge to the power of the trade unions, loosening their hold over the "affiliated organisations" block that constitutes a third of the leadership ballot by letting "registered supporters" vote as part of it.

It is easy to spot the politics. Mr Miliband is desperate to prove he is not in hock to the unions, without whose support it would be his brother David leading the party. With a "winter of discontent" over public-sector pension reform looming, Mr Miliband cannot distance himself far enough from his erstwhile backers, braving heckles at the TUC to reassert his view that the summer's strikes were "a mistake". The changes to the leadership election process are more of the same.

It is a dangerous strategy. By trying to crimp the power that gave him his job, Mr Miliband faces charges of ingratitude, at best; at worst, of undermining his legitimacy as leader. There are also practical questions, such as how many people want to be registered supporters but not members, and how the party will ensure the process is not hijacked by political opponents voting for the least viable candidate.

Even so, the immense symbolic shift Mr Miliband is proposing should not be downplayed. It is a bold move which deserves support. For the majority of voters, trade unions are unrepresentative ideologues, and their grip over Labour is as electorally unpalatable as the commitment to nationalised industry, dropped en route to Mr Blair's first victory.

Mr Miliband is right to take the issue on. If successful, he will significantly increase Labour's chances of electoral success. But given that the proposal will be balloted at the party conference on Sunday, and that efforts to reform the unions' 50 per cent conference vote have been pushed back, there is no easy victory in sight.