IT WAS August 2008 in a giant stadium in Denver, Colorado. A crowd of 70,000 people were waiting to see Barack Obama stride out on to the Democratic Convention platform and accept his party's nomination as candidate for the United States presidency.

In the moments before he arrived, the crowd, electrified by the significance of the event, erupted in a Mexican wave.

Jumping up with them was the British artist Nicola Green, who, commendably, kept her artist's head in the intoxicating atmosphere. In the speeches leading up to this moment, she had sketched the expressive hands of people around her. And, as the wave cascaded through the crowd, she photographed the hands as they waved, pointed, or reached up in expressions of joy and praise.

Green is a London-based artist, whose portraits take viewers by surprise. She discovers character and stories in overlooked human details, such as the hands in the Denver crowd, rather than in more obvious head-and-shoulder compositions.

Her portrait of the children of the writer and film director Hannah Rothschild is a good example. She shows us three pairs of blue school shoes on a hall floor, abandoned after they and their owners have arrived home from school - a daily occurrence for many years. The painting "sums up the carefree spirit of my children perfectly", Mrs Rothschild said recently.

In 2010, Green collaborated with the charity Anti-Slavery International, which works to eliminate all forms of slavery around the world. She produced a stark triptych, "House Slave - Field Slave", which prompts the viewer to consider the misery of modern slaves. Like the Rothschild portrait, the three images focus on human feet, showing "slavery" in the home, and in actual iron chains.

Following the convention of religious icons, the legs and feet of the images are on a field of 24-carat gold leaf, which, the artist says, is there to raise questions of value and identity. How do we value people, and why do we value some people less than others?

GREEN's appropriation of traditional religious imagery arises out of her Christian faith. In fact, her experience as an artist is closely related to her experience of faith: "In my work, I have to follow my heart, and my beliefs, and what I think is important. When I do that, there's no certainty whether I'm going to earn a living from it, whether I'm going to make good work, or how I'm going to do it.

"In that sense, I would equate it to my practice of faith, which is that you follow something you believe in, you think about it, you go through periods of doubt, you don't have any certainty, and you have to follow your inner guidance."

Green's most significant project to date, In Seven Days. . . , is the perfect example of an artist's setting out into the unknown, in an act of faith. It had its genesis in 2005, when her husband, the Labour MP for Tottenham, David Lammy, met the then Senator Obama on a visit to Britain. They forged a friendship, and, when Green was pregnant with their first son, Mr Lammy came back from an American trip to say that Obama was considering running for President.

"As a mother who was about to have a mixed-race son, who would look like Obama, I started thinking about what that might mean for him," she says.

"I looked around at who the mixed-race role-models were for my son, outside of popular culture - and I couldn't see any. I started to think about how my husband and I would talk to our sons about what they could achieve, and what is possible for them in their lives. The idea that this guy who looked like them would shoot for the highest office in the Western world was a big deal."

REFLECTING on how her own story of family and politics meshed with what was happening across the Atlantic, Green realised that she wanted to create a portrait of Senator Obama. But, to do that, she knew she would need to witness the story from the inside, on the campaign trail.

No US presidential candidate has had anything approaching an artist-in-residence before, but she was, first, able to secure his agreement; and, second - and more critically - she was able to persuade his staff to take her work seriously. Initially, they said: "We'll send you some photos," but, through a mixture of charm and persistence, she eventually gained access to the candidate at significant points in the campaign.

For her, the closest analogy to what she was doing is the position of war artist. The wars in Iraq or Afghanistan, for example, are considered so important that somebody is sent who can experience it in real time, reflect on it, and then produce work that is not for the next day's newspaper.

"It means that future generations can look at the event through the artist's eye, sharing in their reflection and sense of perspective," she says.

She made six trips to join the campaign, soon after her second son was born, beginning in Denver and ending with the inauguration in Washington DC in January 2009. Early in the process, she discovered that she did not want to make a one-off portrait of Obama, or even to focus solely on him.

'Every time I went on a trip, I would hear him talking, and watch what he had done in the realms of achieving something impossible. He was an African-American in a former slave-owning nation, standing to be President, and that was massive. In terms of identity, and race, he was representing something truly significant."

GREEN began flying in for campaign events. She sat in the front row and sketched as Senator Obama made speeches. She talked to staff behind the scenes, and citizens in the crowds. She collected magazines and ephemera on the campaign trail.

She took a photograph of him at six one morning in Philadelphia which proved to be one of the most arresting images of the campaign, and which metamorphosed into one of the pieces that she created. And she witnessed the night in Denver when Senator Obama received the Democratic nomination.

"That night was the 45th anniversary of Martin Luther King's 'I have a dream' speech," she says. "The energy there was unlike anything I have ever experienced. There was this connection to that particular moment in history, and also to the future, and what other moments might connect. As human beings, we live for those moments. We live for hope, because if you don't have any hope, then you don't have anything."

She also noticed something that the TV coverage - with its hunger for images of hoopla and hysteria - never truly captured. "What I saw was people queuing endlessly to [attend] rallies, people waiting patiently. I saw this much quieter, patient sort of hope. In the case of the black community, they had been waiting for generations. The women sitting there talked about their own mothers and grandmothers, and it was incredibly moving. They just couldn't believe what was taking place. It made me think about what hope really is."

Each time she returned home, she adopted an almost monastic silence about her activities. She told no one - apart from her husband and parents - that she was going on the trips, or what took place. Even when she returned from election night - where she had talked to Senator Obama just minutes after he gave his victory speech in Grant Park - she did not tell anyone that she had been there, nor what she thought about his election as President.

THIS strong sense of self-denial reveals a powerful link between her faith and her creative work. "I try, as far as I possibly can, to remove my ego and my personal desires from the outcome of my work," she says. "That's my spiritual practice, not just to do with my work."

Green returned to her studio, and began to work from the mountain of primary material that she had collected, to distill just seven images. The images are spare and minimal, each expressing, in an iconic form, the essential themes of the story as the artist experienced it: light, struggle, hope, change, fear, sacrifice/embrace, peace. She conceived In Seven Days. . . as a cycle, without beginning or end.

Like the seven days of the Old Testament creation story, her cycle begins with light and ends with peace. It was not her intention, however, to model the cycle on the biblical account. She sees this story, and her work, as following themes that are universal, regardless of particular faiths. But, having said that, she does see connections with the theme of creation.

"The overarching things I wanted to convey in the work were: how do you achieve something impossible; and what does it mean to do that? The business of achieving anything impossible is essentially a story of creation. That was the most important thing for me to convey to my children; so that future generations who look at this work, whatever they want to achieve, whatever their politics, could look at this story and think about that."

The Sixth Day, Sacrifice/Embrace "Immediately after giving his victory speech on election night, Senator Obama went backstage, and there was no big party, just an overwhelming sense that he had begun the first day of this incredible job," she says.

ONE of the images that stand out in the cycle is "The Sixth Day, Sacrifice/Embrace", in which the silhouetted shapes of Senator Obama's head and outstretched arms float in a square of intense blue. Her desire to transcend the politics and personalities of the 2008 campaign - in order to communicate a story of timeless and universal value - truly succeeds here.

Like all good icons, this is an image that calls for the participation of the viewer, as he or she mentally conjures the standing figure out of the negative blue space. It echoes one of President Obama's main election themes: that it was not just about him, but about all of us, working together for change.

In Seven Days. . . was unveiled for the first time in Britain last month, in an exhibition that is still running at the Walker Art Gallery, in Liverpool. The cycle has been acquired by the US Library of Congress in Washington, and the Metropolitan Museum of Art, New York, which described it as "an artistic and historic masterpiece".

Green is currently working on another project that is as ambitious in its scope as In Seven Days. . . If not more so. Noticing that the leaders of world religions have been meeting over the past decade, and that this is a relatively new factor in world history, she has gained access to some of the meetings as an artist. She wants to understand why they are happening now, and what they mean for us.

Since the fear of difference governs so much of what happens between people of different faiths, it will be fascinating to see the final shape of this new project.

"All my work is basically about exploring identity and difference," she says. "All human stories are actually about how we relate to each other and understand each other's differences. It is the key to how we can live together peacefully."

Simon Jenkins is the editor of Ship of Fools, and blogs at simonjenkins.com.

"In Seven Days. . ." runs at the Walker Art Gallery, William Brown Street, Liverpool, until 14 April. Phone 0151 478 4199.

www.liverpoolmuseums.org.uk

The Bloxham Festival of Faith and Literature opens today at Bloxham School, near Banbury, Oxfordshire. For details, visit www.bloxhamfaithandliterature.co.uk.