Foreign Secretary David Miliband today denied Labour had "run out of steam" and rounded on critics within the party.
On the final day of the Brighton conference, Mr Miliband said Labour had the energy and ideas to win the next election.
And he accused Tory leader David Cameron of showing the "white flag of surrender to euro-extremism" in his own party.
But his harshest words were aimed squarely at those on his own side who have been critical of Gordon Brown's leadership.
"When members of this party, even MPs, say that nothing much has changed, that we could use a spell in Opposition - tell them: don't do the Tories' dirty work for them."
As delegates shouted: "Shame on them!", Mr Miliband went on: "If we do not defend the record no one will.
"Our work is not finished. That is what makes us the agents of change in British politics."
Mr Miliband dismissed the Conservatives as a "bunch of schoolboys" who would make the country a laughing stock in international affairs.
"The Tories aren't a government in waiting," he said. "They are a national embarrassment."
He acknowledged that in the opinion polls the Tories were "back," adding: "And that's our fault."
But he added: "This is not a country crying out for the Tories. It is a country that wants to know what we are made of."
Labour was leading the world on climate change, raising Britain's influence in Europe and reforming the welfare state.
"Don't believe that we have run out of steam. We haven't. So let's show the country that we've still got the energy, the ideas, the hunger, the commitment.
"This is a fight for the future of our country. It is a fight we must win."Reuse content