Last week I walked into a coffeehouse in Chicago. The woman behind the counter had never been involved in politics before. Now she is campaigning for Barack Obama. In Madison, Wisconsin I witnessed swarms of students manning volunteer headquarters and record numbers turning out to vote. As I passed through customs on the way home a security guard high-fived me when I told her I had been visiting the Obama campaign. Something is happening in America: Barack Obama is not just winning primaries, he is surfing a wave of change in politics.

Should the left be filled with optimism or dread about the prospect of an Obama candidacy? So far, in Britain at least, it is not sure. Last week my colleague Will Higham raised questions about Obama’s personal credentials and about his politics. These are worth thinking about carefully, so let’s start with the candidate himself. The first criticism is of a lack of courage. Yet, before his candidacy was even born, Obama proved himself willing to take unpopular decisions when he opposed the war in Iraq. Politically, the decision works in his favour now, but it certainly went against the grain of public opinion in the US at the time – just two years after the September 11 attacks.

Next is the question of resilience. Higham is right to point out that Obama is relatively new to the national stage, but during the race itself, he has already managed to transcend issues relating to his inexperience (he is older than Bill Clinton was when he took office), his background, his experimentation with drugs in his youth, and even the school he went to. His campaign has also held its own against two of the toughest and most experienced political operators in American politics: the Clintons themselves. Against formidable opponents, he appears to be winning out.

Obama has also overcome political setbacks. New Hampshire seriously checked his momentum; South Carolina was dismissed as a demographic one-off. Yet the more voters get to know Obama the more they like him – this was the story of his early career in Illinois and has been the case in this race too. In Wisconsin he won almost every demographic – and has proven himself, thus far, as the candidate the most likely to attract those independents that McCain covets.

And his politics? Sceptics contrasts Obama’s liberal platform with Bill Clinton’s willingness to challenge the left. But, aside from the fact that Obama’s positions are more nuanced than is often suggested, there is an important difference. The key point is that politics is moving on – partly because many of the battles that Clinton waged with his party have been won. The Clinton-Blair generation spent a decade taking on the left. Much of this was necessary, but it can’t go on forever.

For Obama, Clinton’s basic policy framework, not his mode of doing politics, is the starting point. It is a framework that will be tested, nuanced and superseded by new confidence and new ideas. So Obama feels comfortable expressing his support for America’s free market, but also in insisting that huge inequalities are unacceptable. He feels able to move beyond the old vs. new dichotomy. Obama’s promise is that people can achieve more if they are willing to work with one another, and that individualism won’t solve the biggest problems that America faces.

This is where politics must go next. At some point the left needs to break free from a politics beginning to feel outdated and uninspiring. Americans sense an authenticity about Obama: they recognise and respect a refusal to be bound by preconceptions about where Democrats should position themselves. This – and fascinating way in which his campaign has organised itself – lies behind the enthusiasm that his campaign is generating. It is the underlying message and tone of the campaign, not just the soaring oratory, that is inspiring new people to go out and campaign in sub-zero temperatures across America.

Obama represents a generation that is comfortable with the modern world – but also with its own liberalism and desire to do politics differently. The attack ads may yet bring down Obama’s candidacy; he may make a mistake of his own. But, whether or not it is this candidate that succeeds, ultimately the politics that he represents will break through.