It’s a big day for Ed Miliband – the biggest in his short political career.

He’s only been an MP since 2005 – but now he’s got to deliver a speech which could well dictate if the Labour Party has a chance of winning the next UK election in four and a half years.

For the past few months, he’s criss-crossed the country, talking to party activists, articulating why he would be the best person to take over from Gordon Brown. Enough people bought into that vision to elect him leader.

Now in his first major speech since taking charge, he’s got to go beyond that. He’s got to talk to those outwith the hall and explain to the British public what sort of leader of the opposition he’ll make, but much more importantly, what sort of prime minister he would be.

A lot of seasoned political observers are suggesting he’ll never win the keys to Number 10.

Smart – of that there’s no doubt (apparently he can complete a Rubik's cube in less than ten minutes using only one hand). He lectured at Harvard on economics. He was considered a highly impressive climate change minister in the last government. Yet, they say, he’s not strong enough, not decisive enough to be prime minister.

That is perhaps unfair given he took up the challenge against his elder brother David, the former foreign minister and presumptive successor to Brown, and beat him.

But there are many here in Manchester, who while talking up party unity, still believe they elected the wrong Miliband.

When starting with such low expectations – he has the capacity to surprise. And to the wider public, he has to paint the picture of who he is and what he stands for.  If he fails, his opponents will quickly do that for him.

The UK’s Conservative-supporting newspapers have already dubbed him ‘Red Ed,’ suggesting he will move the party to the left of the centre ground previously occupied by Tony Blair and Gordon Brown. The new leader will get a rousing reception at the end of his speech, of that there’s no doubt. But if he fails to deliver a top performance in Manchester on Tuesday afternoon, fails to connect with the British public, politically he could be ‘Dead Ed’.