With her long, straight auburn hair, blue eyes and freckles, Samantha Power looks as though she has stepped out of a photograph of the Kennedy clan. She was born in Ireland, lives in Massachusetts and shares the admiration of America’s royal family for the candidate they regard as the new JFK.

I wonder: could she be a relation of Caroline Kennedy from the wrong side of the blanket?

She laughs: “Maybe it’s the hair. I don’t have any Kennedy blood, stock or power.”

Power came to America at the age of nine with her mother – a doctor who left her father for another man at a time when Ireland did not allow divorce – and has risen to become a senior foreign policy adviser to Barack Obama, the frontrunner in the race for the Democratic nomination for the presidency.

If he continues his dazzling ascent to the White House, she could have a great deal of power and dominion over the rest of us.

Last summer Obama told her she should write a book about her life. “Barack thinks everyone with an interesting life story should write a memoir,” she teases. Power, 37, is engagingly frank, but insists: “I’m more interested in other people’s stories.”

Her Pulitzer prize-winning book on genocide first captured Obama’s attention in 2005 and he may well reward her with a trusted place in his new Camelot. She once referred to herself as the “genocide chick” and, although the subject lacks glamour, it does have its perks. One of these was playing basket-ball with George Clooney – “She’s the best I’ve ever played against,” he says gallantly.

Power, for her part, describes the Hollywood star as a brilliant “one-man diplomat” for Darfur.

“I’d like to say, for the record, that I did not get into issues of genocide for the privilege of meeting George Clooney,” she says with mock solemnity over a coffee in Washington.

“He’s more than a celebrity. He’s doing things that both the US and British governments should be doing. It shouldn’t take a celebrity to go door to door for contributors for troop commitments. I’ve been blown away by him.”

She is also thrilled by the impact Obama is already having on world affairs: “I have a friend who just came back from Burma last week and said all that anybody is talking about on the streets of Rangoon is Barack Obama. What is incredible is how many constituencies he can appeal to, how many boundaries he can cross effortlessly – of race, of age, of geography and of religion.”

“Obamamania”, she believes, owes much to the building of a movement by dedicated supporters. “The only way we were going to win was to have organisers who were willing to freeze their asses off in rural Iowa when it seemed like there was going to be no political payoff. The corollary is that those who are helping Obama do so with quasi-evangel-ical fervour. I think Obama supporters, by and large, do not see this as mere politics. They see this as the future of the world.”

What does Power – who is much sought after by ambassadors because of her closeness to the Illinois senator – think of the report last week that Gordon Brown had badly misjudged Obama’s electoral appeal and was now desperately trying to play catchup after snubbing him in deference to the fading Hillary Clinton? “It’s not fair,” she says staunchly. “He’s not a fair-weather friend who has only just realised Obama is for real.”

At any rate, Sir Nigel Sheinwald, our man in Washington, got in touch with Power almost as soon as he arrived last autumn and met Obama a couple of weeks ago in Maryland.

Hitherto, the principal conduit between Britain and the candidate has been Lord Malloch-Brown, the junior foreign minister, whom Obama came to admire when he was deputy secretary-general of the United Nations.

“He was really taken with him,” says Power, in what will undoubtedly be viewed by American conservatives as a desperately bad sign. “It’s a relationship that has persisted and they have talked a number of times since.”

Power spent her twenties as a reporter in trouble spots such as Kosovo, Rwanda, East Timor and Sudan before helping to found the Carr Center for Human Rights Policy at Harvard University in 1999.

It was as a journalist that she met the subject of her latest book, Sergio Vieira de Mello, a close friend of Malloch-Brown who served as the UN mission chief in Iraq and was a victim of the first big Al-Qaeda suicide attack in Baghdad in 2003.

The dashing Brazilian diplomat was invariably sent to wherever there was a nasty war being fought or a precarious peace to be kept and ordered to make the best of it, which he usually did.

Power and de Mello had dinner together at a seafood restaurant in Croatia when Nato began bombing the Serbs for the first time in 1994. She gave him the opportunity to cancel, but he said insouciantly: “The sky is falling but a man has got to eat, hasn't he? If world war three starts while we’re having dinner, we won’t order a second bottle of wine.”

She was charmed, as were many women who met de Mello; but there was a tough, pragmatic side to the humanitarian that fascinated her. Wherever he found himself, from Bosnia to Rwanda, East Timor, Cambodia and Baghdad, he would do business with warlords and rebels, occupiers and insurgents – whatever it took to rummage up food and shelter for people at risk.

Power has chronicled his life in Chasing the Flame, an intellectually ambitious biography to be published next week.

She writes honestly of his capricious love life – for most of his career he had a loyal wife and two sons quartered in Geneva while he often had girlfriends in trouble zones with him – and movingly details his final hours trapped alive in the rubble of the UN headquarters.

Her passion, however, is the exploration of the art of the possible in foreign affairs. De Mello had little time for UN bureaucracy but loved working in the field. Power is impressed by the “strand of dignity, freedom from fear and impulse towards service” running through his career.

De Mello started out as a “denunciator” who participated in the student battles in Paris in 1968 but became “an accommodator”. Power talked about him at length with Obama while she researched her book and believes that the two men have a lot in common.

“Both guys have thought more about broken people and broken places than just about anybody in public life. Most presidential candidates haven’t lived in broken places. They may have interned in a broken place for the summer. This is in his [Obama’s] blood.”

By the end of de Mello’s life – he died at 55 – she thinks he “may have reached the best balance possible, which was to be willing to walk into a room to see if there is anything you can extract from the unconciliatory”.

She adds: “Neither Sergio nor Barack is ambivalent about the use of force. In East Timor, Sergio was the person who gave shoot-to-kill orders, toughening up the rules of engagement for UN peacekeepers against the judgment of a lot of pacifists at UN headquarters.”

Obama has been criticised for saying he would meet Mahmoud Ahmadinejad, the Iranian president, without preconditions; but Power counters that “definitions of toughness have to be rethought”. It is tougher, she believes, “to be in a room with Ahmadinejad than lobbing verbal hand grenades against him from 5,000 miles away” – a position now being adopted by some neoconservatives who are urging President George W Bush himself to open talks.

Power shocked some friends by opposing the invasion of Iraq. “Some people said, ‘How could you write about genocide and not support the war?’ It felt like one was sentencing the Iraqi people to life imprisonment under a brutal regime.”

She decided, however, that there was no evidence that Saddam Hussein was planning a new round of slaughter and the costs and benefits of war were not worth the risk. Now she admits the question is: “If we leave, could it get worse?”

Power believes that Obama would do all he could to prevent large-scale civilian deaths by a meticulously planned withdrawal from Iraq. “If a systematic campaign of genocide were to unfold, he would try to get other countries to join the US to go back in order to stave it off. What you are talking about is a massive crime against humanity and the hope would be that humanity would step forward.”

She knows, however, that “with the record of stepping forward in Rwanda or Darfur or in the Holocaust, it is going to be very difficult”.

Just as de Mello’s good intentions did not save him from Al-Qaeda’s nihilism, so Obama’s message of hope could be met with cynicism. So far he has triumphed over every expectation. If his luck holds, perhaps Obama-mania could sweep the world.

Chasing the Flame by Samantha Power is published by Allen Lane on March 6 at £25

Times Recommends

• Times Charity Appeal: SolarAid
• Is it Black Monday all over again for Dubai?
• Behind the scenes at the biggest deal