On 30 October, Barack Obama addressed a rally of 30,000 people in Chicago, as part of a last-ditch attempt to save his own former Senate seat of Illinois. It was his first public appearance in his hometown since his 2008 election-night victory rally. "In three days, you have the chance to once again say what?" Obama shouted into the microphone, cupping his hand around his ear and leaning forward. "Yes, we can!" the crowd shouted back.

On 2 November, the voters delivered a different answer to the president's question: "No, you can't." In one of their worst election defeats in a generation, the Democrats lost control of the House of Representatives to a resurgent and reactionary Republican Party and only narrowly retained control of the Senate. Humiliatingly, the president was unable to save his own former seat.

Much ink has been spilled over the reasons for Obama's decline. The list is as long as it is depressing: the ongoing economic downturn and the administration's failure to protect jobs; a relentlessly negative and uncompromising opposition; a hostile and often hysterical media; the surprising inability of Obama himself to communicate his message to the voters.

He's not the messiah

Above all else, Obama raised expectations to unprecedented levels. The messianic "Yes, we can!" candidate of the 2008 campaign trail became in office a cautious and overly deliberative pragmatist. Despite being denounced by opponents as a "socialist", Obama failed to offer a convincing, left-wing economic populism to counter the right-wing, anti-state populism of the Tea Party. He couldn't mobilise the 13 million "virtual" activists on his much-vaunted email list to take to the streets against the opponents of health-care reform. In the words of a Labour strategist who has worked with the Obama White House: "He was expected to stand up for the little guy against the vested interests. He didn't."

Instead, Obama and his aides trained much of their verbal firepower on their own supporters. The president's former chief of staff, Rahm Emanuel, dismissed liberals concerned about the administration's health-care bill as "fucking retarded"; the White House press secretary, Robert Gibbs, said left-wing critics of the president "ought to be drug-tested". Obama did not just neglect his base; he abused it. Is it any wonder the Democrats didn't turn out for him in as huge numbers on 2 November as they had only two years before? Or that 47 per cent of them say Obama should face a primary challenge for the presidential nomination in 2012?

So what lessons are there, if any, for Ed Miliband? Labour strategists, as well as shadow ministers, are analysing the US midterm results. Like the American president, the Labour leader ran as an insurgent, an outsider, against his party's establishment candidate, inspiring younger activists to join his campaign and raising money via online donations. He advocated a much tougher stance on the banks, distanced himself from the Iraq war and spoke incessantly of the need for "change".

So is he as vulnerable as Obama has been to disillusionment and cries of "betrayal" from his own centre-left base? "If you run a sort of pseudo-Bobby Kennedy campaign of hope and change, you're going to encounter disenchantment," says a former Downing Street aide, who has worked closely with Miliband in the past. "Ed is going to have to show people he is an idealist as well as a pragmatist. If he can't do it, he's not the right guy." He adds: "I'm worried about how quickly a sense of real drift will set in unless he starts to interest people."

Obama, says a shadow cabinet minister, allowed himself to be cut off from the "netroots" style network of volunteers, activists and community organisers that he had helped to create as a candidate. Miliband cannot make the same mistake. "Obama failed to sustain himself in office by continuing to build and inspire a radical, progressive movement - and so the movement atrophied," he tells me. "Ed has to build a mass movement with a very clear vision for changing Britain.

Supporters of the Labour leader claim his radicalism is undimmed; "our agenda," argues one, "is "Thatcher-esque in its ambition". Tackling inequality and low pay, as well as exploring the limits of financial markets and the City of London, are themes that the Labour leader rightly intends to revisit in the coming months. Others point to Miliband's impassioned critique of the coalition's proposed cap on housing benefit. "There aren't any votes in defending people wrongly described as 'scroungers' in the Daily Mail," says a source.

Meanwhile, inside Miliband's offices in parliament's Norman Shaw South building, his aides are in constant discussions over how to renew and rebuild what their leader referred to as a "hollowed out" party during his election campaign. References to "community-organising" and "reaching out" to NGOs abound. "We have a party, with members. Obama doesn't," I was told. "That's what we're going to build on and transform."

Voice of the little man

But is the leader's inner circle, as it is currently constituted, up to the task? I sense some may be more cautious than Miliband himself wants to be. "He has a very small team, some of them quite junior, all of them working 18-hour days," says a friend of the Labour leader. "Ed's team had a plan to win the leadership election but they didn't really have a plan for what they would do if they won. David [Miliband] and his people had a much more developed plan for the party. I'm not saying it was the right plan but they had one."

It would be a mistake, however, for Miliband's opponents to underestimate him. His decision to challenge and defeat his elder brother and then deny the jobs of chief whip and shadow chancellor to Nick Brown and Ed Balls, respectively, shows that he can be bold and decisive.

Nonetheless, Miliband has to speak out more often and be unafraid of expressing anger or outrage about vested interests - be they financial or political. "There has been a diffidence to his start," says a friend. "He has to change gears and get going." The lesson from the United States is that this is not a time for diffidence; in a period of economic dislocation, the left cannot afford to let the right surf the inevitable wave of anger, insecurity and discontent. Miliband must become the voice of the "little guy". And so, for that matter, should Obama.