Ed Miliband has made a speech that marks the most emphatic break yet with New Labour. Tony Blair could never have made it because he would have been uneasy about the arguments. Gordon Brown would never have made it, even if he believed in the arguments. Both would have worried about appearing too left-wing. The contrast with Blair is especially marked. The former Labour leader used to claim his boldness while making cautious arguments that rarely challenged orthodoxy. Miliband never made claims for his political courage, but he was being brave.

Contrasts with a three-times election winner are not necessarily flattering. Blair mesmerised the hall and the wider audience with elegantly constructed speeches, even when he was waffling about a country re-born. Each sentence in a Blair speech was constructed to appear daringly radical while reassuring virtually every voter in the country and a Conservative-dominated press. He knew how to win elections even if he was not inclined to make fundamental change. Miliband did not hold his audience at times and the speech was structured awkwardly. Recognising a move to the left, the Conservative newspapers will be dismissive.

But Miliband makes a calculation that will determine the fate of his leadership. In the midst of an historic economic crisis, he seeks to move the centre ground leftwards with a genuinely new message about the role of government, the need for responsibility at the top as well as the bottom, a fresh onslaught on vested interests, a fairer system of taxation and demand for wider access to the elite institutions, including universities. In making his case he is unlikely to concede that he is moving his party to the left, but he is.

He does so partly by portraying David Cameron and George Osborne as part of the old order, incapable of adapting to changing times and reactionary in their instincts. "How dare they say we're all in this together?" he asked as he highlighted their commitment to retain new taxes on the so-called squeezed middle while seeking to scrap the top rate on higher earners. If poorer students get the grades, Miliband wants them in Oxbridge – a hint of an old populist campaign he and Ed Balls felt Gordon Brown should never have pulled back on. He implies new lines of accountability to ensure that the recklessness of the banks and the accompanying rewards would not be allowed to happen again or apply elsewhere. In all of this he hopes to create space for a new populist message from a leader on the centre left, as Margaret Thatcher managed to do on the right in the 1970s when she insisted she was for the workers and not the shirkers, an iron lady ready to stand up for hard-working families and to get the crippling state off their backs.

One of the problems with Miliband's speech was the lack of equivalent accessible phrases, ones that light up a long address. He is not an exuberant public performer and perhaps such soundbites would strike a false note, but without them his speech meandered at times. As he spoke it did not feel especially significant, even though it might prove to be.

That will depend on whether Miliband can flesh out an argument with detailed policy. His speech had similarities with the one made by Cameron early in his leadership in which he set out why he was an optimist, wanting to let the sun shine through. I recall writing against virtually every paragraph of that address a one word question: How? Miliband's speech was similarly light on policy. What form would his more active state take? The biggest challenge always in politics is to link values with precise policy commitments that have broad appeal. In identifying the ending of the old lightly regulated era, Miliband moves with the tide of history, a break at least as big as the collapse of the old corporatist consensus in the late-1970s. That is the easy bit. Coming up with policies that unite his party and appeal to voters is much more challenging.

Speeches that take the form of a broad argument rarely change the political weather. Blair's "forces of conservatism" address during his first term, Cameron's "let the sun shine" speech and now Miliband's call for new values all lacked clear definition, perhaps because each speaker was a little fearful of what they were trying to define. Miliband strides away from the past, distant terrain that traps others. His chosen path is testing but he is armed with values of broad appeal. Next he must find the policies and convince the wider electorate to join him. His party will know by the time of next year's conference whether he has the titanic qualities required for such a journey.

Anatomy of a speech

The tie A nicely groomed, carefully combed, strictly controlled appearance - but with a black tie. What was he mourning? Everything he used to be.

TV screen failure Little did Ed know it, but he was cut off from the outside world for seven minutes as the television feed from the hall failed. Ironically, with Labour contemplating at least three and half more years in opposition, the problem was a power surge.

Mrs Miliband "That was so great Ed, but who are all these people? It's so exciting, can we come again next year?" Out of camera shot. In the audience the leader's press man Tom Baldwin laughed with, talked to and occasionally stroked Labour conspirator Derek Draper. It's impossible to say which of them could be improved by the other's influence.

The music One song has made a regular appearance over the years: Just Can't Get Enough. The electorate indicated last year it had had quite enough. The song selectors must mean Labour just can't get enough (power, position, attention, our money).

Simon Carr and Nigel Morris