In the tough battles ahead to secure the Democratic nomination, the strategists behind the campaign to send Barack Obama to the White House say their best weapon to lure voters is exposure.

"Obama represents the change the people are looking for," said David Axelrod, the Illinois senator's campaign chief-of-staff, as he yesterday savoured victories in a majority of Super Tuesday states. "The more they are exposed to him, the more they get his message."

Voters in Washington, Louisiana, Nebraska and Maine can expect him next, though he might have to campaign all the way to the party's convention in Colorado in August if the primaries to come fail to produce a clear winner by then.

He will deliver his message, if the momentum of recent weeks is sustained, to swelling crowds of enthusiasts who have given a revivalist flavour to Mr Obama's political odyssey.

Its substance, long on inspiring rhetoric but sometimes short on detail, according to his opponents, will not change. "It's the message that's going to lead him to the nomination of this party," Mr Axelrod says.

The essence of Mr Obama's grassroots bid is that the nature of US politics and the electorate, bruised by concerns over security, the economy and a divisive war in Iraq, has fundamentally changed.

In speeches he has promised an alternative to the machine politics of both parties in which he says Washington lobbyists wield more power than the voters.

"We are the ones we have been waiting for," he told his euphoric supporters on Tuesday night. "We are the change that we seek."

But Mr Obama still has a hill to climb. Hillary Clinton took a majority of delegate votes at stake this week, although she ended up with less than the 100-vote advantage that the Obama campaign said it would regard as a satisfactory outcome.

Some of his backers believe there has been a sea-change in US politics that will play to his advantage.

Jesse Jackson, the first African-American to make a bid for the White House, told the Financial Times: "He's winning the white vote in the south. It's common for blacks to vote for whites. For whites to vote for blacks is a sign of a maturing America."

The Rev Jackson said he believed the Illinois senator had the means to secure the nomination: "He has the presence. He has the message. He has the money, the machinery and the timing."

It remains unclear, however, whether he has the across-the-board appeal to challenge the more experienced Mrs Clinton.

Super Tuesday indicated Mr Obama had consolidated the African-American vote and garnered the support of young people, independents and even disaffected Republicans. He harnessed an anti-war movement and showed it to be more potent than political pundits had predicted in a campaign they said would be dominated by the economy.

They might still be proved right. Among blue-collar workers in New Jersey, Hispanic wage earners in New York and Asian shopkeepers in California - communities obliged to study the state of their pocketbooks rather than foreign policy when they vote - Mrs Clinton still has the edge.

Mr Obama has captured the hearts of idealistic youth and of a white middle class shamed by the decline in US standing in the rest of the world during the Bush years. Combined with a reliable base among African-Americans and a respectable share of women voters in the first presidential race to feature a woman, that might be just enough to hand him a ticket to the White House.

David Plouffe, who crunches the numbers for the Obama campaign, thinks it is feasible. "From Colorado and Utah in the west to Georgia and Alabama in the south to Senator Clinton's backyard in Connecticut, Obama showed that he can win the support of Americans of every race, gender and political party in every region of the country," he said.