New party leaders, as Ed Miliband joked in his conference speech on 28 September, are never short of advice. MPs, activists, columnists and bloggers line up to offer their words of wisdom - from tips on political strategy to guidance about hairstyle and clothing.

On the night before his big speech, Miliband was surrounded by a group of close aides and loyal MPs in the "leadership suite" of the Midland Hotel in Manchester. Fuelled with caffeine, "Team Ed" - led by Stewart Wood, the clever Oxford academic and former adviser to Gordon Brown - had been working for 48 hours straight on the text of the address. As the finishing touches were being applied, various people in the room began offering conflicting opinions on the tone they believed Miliband should strike the following day. Exhausted and exasperated, the newly crowned leader of the opposition turned to one of his advisers and said: "Let Miliband be Miliband."

The line is a reference to the award-winning US television series The West Wing, which portrayed a fictional, liberal US president and his idealistic White House staff. Towards the end of one particularly memorable episode, the president, Josiah "Jed" Bartlet, is confronted by his chief of staff, Leo McGarry, who challenges him to act boldly, take his advisers "off the leash" and "raise the level of public debate". "You have a strategy for all this?" asks the president. McGarry responds by scrawling a note on a legal pad: "Let Bartlet be Bartlet".

No right turn

The new Labour leader has long been a fan of American television. As a teenager growing up in the 1980s, he was obsessed with the soap opera Dallas. These days, his favourite show is the US comedy-drama series Desperate Housewives. But Miliband's decision to quote from The West Wing is as instructive as it is amusing. It suggests that he understands the need to define himself, rather than allow others - the media, the Tories, Blairite supporters of his elder brother - to define him: whether as the hapless "Forrest Gump" or the unreconstructed "Red Ed". "Come off it," he defiantly told a packed conference chamber, to rousing applause. "Let's start to have a grown-up debate in this country about who we are and where we want to go."

This was the speech that we had been waiting - longing - for a Labour leader to deliver to his party: intelligent, honest, bold and solidly social-democratic. It was not a speech that Tony Blair or Gordon Brown would have given, nor was it the speech his brother, David, could have given.

The Iraq war was "wrong", the Israeli attack on the Gaza aid flotilla "so wrong", the embrace of markets "naive" and the treatment of civil liberties "casual". He would, Ed Miliband announced, be a "responsible" leader of the ­opposition and abandon the disgraceful New Labour tactic of attacking the Tories and Liberal Democrats from the right on penal policy, counterterrorism and the war in Afghanistan. Optimism, not pessimism, was the leitmotif.

In the junior Miliband, Labour has a leader with ambitions to reshape the "centre" ground of politics. As he told me in August, in the midst of his leadership campaign, the Conservatives and their allies in the press "want to define the ground of politics on the right. It is part of our job to define the ground of politics in a different place."

He has a fight on his hands. The militant Blairites are disappointed at the defeat of their candidate, David Miliband, at the hands of his younger brother. They descended on the bars of Manchester to echo the warning of their guru, Tony Blair, that it would be electoral suicide for the party to move even "a millimetre from New Labour".

One ally of David Miliband told me at the New Statesman reception at Manchester Town Hall on 26 September, the day after the result was announced: "Ed's got to move right. He has to and he will."

Prior to the speech, supporters of the new leader to whom I spoke were worried that the younger Miliband, fresh from his victory, would indeed tack not just to the centre, but to the right. Few had forgotten how his two immediate predecessors could never resist triangulating on issues such as immigration and deregulation in their deluded attempts to ­appease the likes of Rupert Murdoch and the Daily Mail editor, Paul Dacre.

But Ed Miliband's conference speech, with its repeated references to a "new economy" and the "good society", to "love and compassion" and "solidarity", suggests that his centre-left rhetoric is a by-product of a sincerely social-democratic ideology, rather than a cynical ploy to win the Labour leadership. To borrow a line from Blair: it's worse than the new leader's critics think - he really does believe in it.

Sink or swim

Can it last? Will Ed Miliband stick to a progressive, Keynesian position on deficit reduction? And continue to advocate popular and progressive policies such as a tax on bankers' bonuses and a living wage? Key members of Team Ed tell me that they are well aware of the numerous threats and challenges that the new leader faces from inside, as well as outside, the party. "We are building a ring of steel around him," says a senior Labour MP and long-time ally of the younger Miliband. "We won't let the Blairites get to him."

The "phoney war" of the interminably long Labour leadership contest is well and truly over. The battle, between Labour and the Conservative-Liberal Democrat coalition, now begins. Ed Miliband had less than three days between being announced as the new Labour leader and having to deliver the "speech of his life" to the party faithful in Manchester. He will have less than 13 days to agree Labour's crucial policy on spending cuts with his new shadow chancellor, as the results of the shadow cabinet elections won't be declared until 7 October.

As with Bartlet, much depends on the boldness and distinctiveness of his political judgement. He cannot dither or prevaricate; nor can he afford to alienate the younger, idealistic supporters who joined his campaign over the summer. "Ed will either be the best leader Labour has had in a very long time, or the worst leader," says a shadow minister who knows him well. "He will either soar or crash."

Or, as Miliband himself recognised in his speech in Manchester: "Politics has to be about leadership or it is nothing."