The Brazilian president, Luiz Inácio Lula da Silva, has a compelling personal and political biography. One of eight children, he could not read until he was 10, left school soon after and by the age of 12 was working as a shoeshine boy. Lula was instrumental in setting up his own leftwing political party, the Workers party, risked jail as a trade union organiser during the dictatorship and ran for president three times before he was finally successful in 2002, capturing the imagination and hopes of many Brazilians - albeit with a vastly watered-down programme.

Having finally won the presidency, a moment many of his supporters thought would never happen, he was then cruelly mugged. The invisible hand of the market grabbed him on his way to the inauguration and shook what was left of the socialism out of him. In the three months between his winning the vote and being sworn in, the nation's currency plummeted by 30%, $6bn in hot money had left the country, and some agencies had given Brazil the highest debt-risk ratings in the world.

"We are in government but not in power," said Lula's close aide, Dominican friar Frei Betto. "Power today is global power, the power of the big companies, the power of financial capital."

In any democracy the link between the electoral and the political is essential but not inextricable. Between the trappings of democracy and the trials of legislating, there is power. The balance, distribution and strategic exercise of it shapes the relationship between expectations and possibility, marking the distinction between being the will of the people and the work of government.

It is the very tension that lies at the heart of Barack Obama's candidacy and the energy it has unleashed. To attract 75,000 people to a rally, as he did in Portland, Oregon, recently, shows immense drawing power. The question is, what to do you say to them when they get there?

On the one hand, he has managed to articulate the aspirations of many people from whom we previously heard little, if anything, in American politics and mobilise them into a formidable voting bloc. On the other, the progressive forces that have gathered around him have now wedded themselves to a decidedly mainstream, tepid political agenda. Lula, at least, resisted the assaults on his base; Obama, at times, appears to embrace them.

That an Obama victory would mark a radical improvement on George Bush and be far preferable to John McCain, there can be no doubt. Electorally, that is important. But politically, it leaves open the question of whether he is prepared to adopt an ambitious programme that can address the mess he will inherit. Politically, this question could have been asked of any of his main Democratic rivals in the primaries, none of whom pursued radical agendas. But electorally, more has always been claimed of his candidacy and more has also been expected of it.

Let's start with the obvious. Electorally, Obama's nomination marks a truly exciting and historic moment in US history. In a nation that prides itself on relentless progress and social meritocracy, the symbolic importance of a black president can be over-exaggerated. But that does not mean it should be dismissed. He was born before he had the constitutional right to vote (secured by the 1965 Voting Rights Act), to mixed-race parents who did not have the constitutional right to marry (the supreme court only legalised miscegenation in 1967). His campaign represents a milestone in America's scarred racial landscape. Of the 10 blackest states, he won nine; of the 10 whitest, he won seven. He has broken a mould. And it can't be reset.

Moreover, his candidacy has sparked a realignment in the coalition of forces that comprise the Democratic party, by rousing dormant and ignored constituencies - notably the black and the young. The Democrats have consistently won the youth vote since 1992 but have failed to galvanise a sufficiently high turnout for it to be decisive. The black vote, on other hand, has long been both crucial and taken for granted. The party has only won the majority of the white vote in a presidential election once since the second world war. In the past, both groups were at best treated as junior partners and at worst simply forgotten.

Not any more. Obama's campaign helped raise the share of young people's (18-29) votes in the Democratic primary by more than 50% compared with 2004. Between them, the young vote and the black vote comprised 28.8% of the Democratic primary electorate in 2004. This year it was 35.1%. Their swelling numbers and contagious enthusiasm will give them considerable leverage within the party.

If - a big if - he can maintain the rest of the Democratic base, this could bring into play states like Virginia and North Carolina, which the Democrats have not won since 1964 and 1976 respectively. His candidacy could set an earthquake under the established electoral map.

He has also transformed the model for funding, creating a broad popular base of small donors. Unprecedented numbers of people have invested in him. The question is whether they will see a return.

The earliest signs have not been promising. The day after he clinched the nomination, he went with Hillary Clinton and McCain to genuflect before the pro-Israeli lobby to declare himself a "true friend of Israel". But good friends sometimes tell each other things they need to hear, even if they don't want to. America's uncritical support for these past eight years has been deeply unhealthy and has been neither in the interests of America or the Middle East. Correcting it is central to the US improving its dire standing in the Arab world and gaining international credibility in general - two things his supporters crave. Instead he pandered, stating that "Jerusalem will remain the capital of Israel, and it must remain undivided", and promising not to withdraw from Iraq until the conditions on the ground were right.

Meanwhile, the economy continues its precipitous decline. Unemployment is increasing, the dollar is slumping and inflation remains high. House prices are nosediving and fuel prices are skyrocketing. Each month more and more Americans find themselves at the precipice. One in 11 mortgages are either in arrears or foreclosure. More than one in six homeowners has negative equity or no equity in their house. By June, claims Moodys, that will rise to one in four.

Yet Obama refuses to call for a moratorium, an interest rate freeze or substantial government spending, preferring instead a tax credit for homeowners that would amount to little more than about $500, beyond which only some borrowers could get more help. Over-represented among these sub-prime borrowers are the very African Americans who have propelled him to victory.

The great thing about Obama's candidacy is that he has raised expectations about what American can be and do in a way that nobody else has or could in recent memory. Whether they develop into pressure or descend into cynicism is an open question. Will he be a vehicle for their hopes, or will they be a vehicle for his political ambition? The two are not mutually exclusive. But their connection is far from assured.



g.younge@theguardian.com