"She was a victim of both the forces of history and the forces of destiny," said King. "She had been tracked down by the zeitgeist - the spirit of the times." The reality was somewhat different. Parks was no victim. The zeitgeist did not track her down; she embodied it. She had a long history of anti-racist activism and had often been thrown off buses for resisting segregation. Far from being a meek lady in need of a foot massage she was a keen supporter of Malcolm X, who never fully embraced King's strategy of non-violence.

"To call Rosa Parks a poor, tired seamstress and not talk about her role as a community leader and civil rights activist as well, is to turn an organised struggle for freedom into a personal act of frustration," writes Herbert Kohl in his book She Would Not Be Moved.

The story of collective struggles is all too often filtered through the experience of an individual. In a bid to render the account more palatable and popular, the personal takes precedence over the political. As a result the story may reach a wider audience; but by the time they receive it, the agendas and the issues involved have often become distorted - to the detriment of both the individual and the movement.

The story of Cindy Sheehan, the 48-year-old woman whose son Casey was killed in Iraq in April 2004, is one such example. Until late last week, Sheehan was camping outside President George Bush's ranch in Crawford, Texas, demanding to see the president. "I want to ask him, why did my son die?" she told the Guardian. "What was this noble cause you talk about? And if the cause is so noble, when are you going to send your daughters over there and let somebody else's son come home?"

Sheehan, who has met the president once before and was not impressed, had planned to stay at what became known as Camp Casey for the whole of August but had to leave on Thursday because her mother became sick.

With the help of PR consultants she was packaged as a grieving Everymother who wanted answers. Capturing the public imagination, over the past two weeks she has been a regular feature on US cable and network news, the letters pages and newspaper editorials. In turn, she has re-energised the anti-war movement. On Wednesday, thousands of people across the country attended 1,627 vigils in solidarity with her cause.

Her popularity has made her a prime target for the right. One commentator on Rupert Murdoch's Fox channel branded her a "crackpot"; Christopher Hitchens derided her for "spouting piffle" and lambasted her protest as "dreary, sentimental nonsense". Talk-radio king Rush Limbaugh said her story "is nothing more than forged documents - there's nothing about it that's real". The backlash continued this weekend with the launch of a "You Don't Speak for Me Cindy" tour heading for Crawford with the support of rightwing talk radio hosts, to set up a pro-war camp.

The focus on Sheehan's personal loss is indeed problematic. Bereavement, in and of itself, confers neither knowledge nor insight - only a particular sensibility that might lead to both and a compelling personal narrative through which to articulate them. To define her as a mournful mother, while ignoring that she is a politically conscious, media-savvy campaigner, which she has been for quite some time, does neither her nor her cause any favours.

Indeed, those who focus on Sheehan's woes, whether they support or attack her, miss the point entirely. Had she come to Crawford at Easter, she would most likely have gone unnoticed. The reason she has struck a chord is not because of the sorrow that is personal to her but because of the frustration she shares with the rest of the country over Iraq. That is also why the right have attacked her so ferociously and so personally.

But unlike the Democratic presidential candidate John Kerry in his swift boat, Sheehan will not be blown off course quite so easily. The public mood in America is shifting consistently and decisively against the war and Bush's handling of it. Gallup has commissioned eight polls asking whether it was worth going to war since the beginning of the year: every time at least half have said no. For the first time, most people believe the invasion of Iraq has made the US more vulnerable to further attacks. The number of those who want all the troops withdrawn remains a minority at 33% - but that is double what it was two years ago, and still growing.

The reason Sheehan has become such a lightning rod is because that mood has found only inadequate and inconsistent expression in Congress. It has been left to her to articulate an escalating political demand that is in desperate need of political representation. This marks not only a profound dislocation between the political class and political culture but a short circuit in the democratic process. The mainstream has effectively been marginalised.

This is not particular to the US. In Britain, the view that there was a link between Iraq and the London bombings was shared by two-thirds of the population, but the handful of politicians who dared to mention it were shouted down in parliament and vilified in the press. In Germany, all the main parties support the labour market reforms that will cut welfare entitlements and reduce social protection, even though most of the population do not. But what many "centre-left" politicians regard as electoral expediency is actually becoming an electoral liability. Evidence exists that support for more radical stances is there if only they had the backbone to campaign for it.

In Germany, a new leftwing party combining ex-communists and disaffected Social Democrats is attracting 12% in polls and could yet rob the right of an outright victory next month. This month, in a congressional byelection in southern Ohio, Paul Hackett, a marine reservist who recently served in Iraq, stood for the Democrats on an anti-war platform. In a constituency where the Republicans won with 72% of the vote nine months ago, Hackett branded Bush a "chicken hawk". He won 48%, turning a safe seat into a marginal.

Sadly, such examples are all too rare. Sheehan has revealed both the strength and the weakness of the left. We have a political agenda that can command considerable mainstream support; yet we do not have a political leadership willing or able to articulate those agendas. We wield political influence; we lack legislative power.

g.younge@theguardian.com