Barack Obama does not usually have to ask twice to get what he wants. But that is what he had to do to get Joseph Nye as a campaign adviser on foreign affairs.

It is only in the past few weeks that the former dean of Harvard's John F. Kennedy School of Government has agreed to join the Democrat presidential nominee's team. An earlier invitation, issued while Obama was fighting Senator Hillary Clinton for the Democrat nomination, was declined. Although Nye voted for Obama in the Massachusetts primary, he explains, "I know and respect Senator Clinton and I've worked with her and with former President Clinton in the past, so I did not think it was appropriate." With the nomination settled, that objection no longer applies.

If such old-fashioned gentlemanliness seems incongruous in the full-contact context of US presidential battles, it reflects Nye's manner - cordially courteous in the style of his generation of American academics - and belief that principle matters in politics.

One of his favourite stories from his own time in government concerns Secretary of State Cyrus Vance's principled stance over the ill-fated attempt to rescue the hostages imprisoned in the US Embassy in Iran. "He disagreed with President Carter's policy. He told Carter he was going to resign, but not before the raid took place, as he did not want to do anything that might reduce the likelihood of its succeeding. He said he would resign - and duly did - after the raid, whether or not it succeeded."

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Although Nye is in his early seventies, there are no signs that he is slowing down or resting on the laurels of a lifetime in academia and government. If there were, he would hardly be in demand from a presidential candidate running on a change ticket. He continues to teach two terms a year at Harvard University while spending the other in Oxford, has added a blog to his range of academic and journalistic publications, and is still refining his best-known concept "soft power". His new book, The Powers to Lead, examines how soft power - which he defines as "the ability to get what one wants through attraction rather than using the carrots and sticks of payment or coercion" - applies as much to individual leaders as to nations. It is also crucial to the task that an incoming president - whether Democrat or Republican - will need to address, namely that of rebuilding America's reputation in the world.

The idea of soft power was developed in the late 1980s and early 1990s as a response to Paul Kennedy's book The Rise and Fall of the Great Powers: Economic Change and Military Conflict from 1500 to 2000, which Nye argues "concentrates too much on economic and military power and not enough on other sources, which for the US range from Hollywood to Harvard".

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He and Kennedy have maintained mutual respect through two decades of vigorous but amicable sparring, most recently in lectures last term at the London School of Economics. Kennedy calls Nye "one of the most brilliant speakers we have, with a remarkable ability to analyse and explain complicated issues".

Like many American academics, Nye has alternated teaching and research with spells in government, serving in foreign affairs and national security posts in both the Carter and Clinton administrations. There is little doubt, though, that he regards himself as primarily an academic. During the initial interview with Times ÖйúAƬ, he suggested that a serious terrorist incident between now and the presidential election in November would benefit Republican candidate John McCain. Shortly afterwards, Charles Black, a full-time member of the McCain team, prompted outrage - and a distancing by his candidate - with a similar observation.

When asked about this, Nye points out that what is acceptable as academic observation may be much less so when put forward in a partisan context. "Charlie Black's comment may have been politically incorrect, but I continue to think that it is analytically correct. That's the nice thing about being an academic."

Nye concedes that McCain is "an impressive and appealing person". He does not expect a particularly partisan election and points to a number of areas of apparent cross-party consensus. Both sides seem to accept, for example, that unilateralism, based on the belief that the US is strong enough to do what it likes, has devastated its ability to influence other countries through "soft power".

Earlier this year, Nye was the Democrat counterpart to Richard Armitage, Deputy Secretary of State in George W. Bush's first term and an important foreign affairs adviser to McCain, as co-chairman of a bipartisan "Smart Power" commission to look into ways of integrating soft power with the harder variety represented by economics and military might.

In a body equally divided between the two parties, Nye saw very few partisan divisions. "What is intriguing is how little difference there is. We all feel that any new administration will have to put a high priority on multilateralism, on strengthening international institutions and providing global public goods, taking the lead on health, climate change and so forth. People have learnt the lessons of the past seven years, but at a very high price."

In particular, he argues: "We have to stop exporting fear. There are a number of steps any new Administration can take, notably closing Guantanamo Bay. There are always difficulties in being the big kid on the block, but we're in trouble if we are seen - as we certainly are at the moment - as the bully."

It is not as if the US has not been here before. Its international reputation was almost equally tattered in the aftermath of the Vietnam War but, as Nye recalls: "We changed our policies toward Vietnam, and developed policies that stressed human rights. Bipolarity and a Soviet threat provided a favourable context for this, but not the whole answer."

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He believes the present situation is similarly retrievable. "While the US is undoubtedly internationally unpopular at the moment," he says, "that is a reflection of the policy of this Administration, rather than a rejection of American values. How we disengage from Iraq, how we handle Iran and how we make progress on the Israel/Palestine peace process will be important for the Muslim world. Also important will be changing to a leadership position on global climate change, and the development of a more multilateral style in our approach to issues."

While domestic economic issues currently dominate the agenda, Nye expects Iraq and Iran in particular to also play an important role. "[The candidates] differ most on when to leave Iraq and about talking to Iran without preconditions. I agree with the Obama position that we should enter a broad-based discussion without preconditions. He has been criticised for 'premature summitry' by McCain, but the discussions need not start at the summit."

Both the original concept of soft power and Nye's more recent application of it to individual leadership are informed by his experience. "Studies of leadership have tended to overemphasise the coercive element," he explains. "Coercion may be part of it, but if you understand power as the ability to get what you want, then attracting people to you is also important. I found when I worked in government that there were people over whom I had no power that I needed to get on my side if I wanted to get things done, and I had to learn how to attract them to support me."

On the basis of Obama's bestselling autobiography, Dreams from My Father, as well as his campaign, Nye claims that the Democratic Party's presidential candidate has the background, experience and temperament to win people over and preside over America's essential change of direction.

"Elements in his background - he has an African grandmother and grew up in Indonesia - make him particularly capable of connecting to the rest of the world. He has thought through the question of identity, which is a fraught problem for America, in a way that few people have done.

"He realises that in many people's eyes he is black, but he grew up in a white home. He was simultaneously capable of editing the Harvard Law Review and of going to work for black people in South Chicago.

"Working through those issues of identity has given him a knowledge of himself, an 'emotional intelligence', that is very important for a leader. It has given him an ability to master his own emotions and reach out to other people in a way that attracts them to him."

So will Obama get the chance to deploy those qualities as president?

"I think he has a better than even chance of winning," says Nye, adding that how the economy fares and the trade-off between Obama's evident attraction for groups normally unenthused by politics and covert racists shifting away from the first black nominee from a major party will help determine what happens on 4 November.

"Nothing is very predictable in American politics," he points out. "A year ago a Republican consultant told me it would be Edwards versus Romney, and he gets paid for that sort of thing."

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