Part of it stems from an intense internet campaign. The senator's website lets potential donors individually blog, organise and create their own donation pages and, for months, his campaign has been interacting with potential voters and donors beyond anything ever tried before.

But beyond that his team has worked more discreetly among traditional sources of Democrat money. Hollywood and Wall Street have been tapped, including the connections of his advisers veteran investor Warren Buffett and the former treasury secretary, Robert Rubin.

All up, he's received donations from 3.1 million people - a significant increase on the 2 million donors registered by President George W. Bush in his victorious campaign for a second term four years ago. That effort was seen at the time as a sign of Mr Bush's considerable campaign skill and organisation, and the talent of controversial strategist Karl Rove in energising the religious right.

Senator Obama's reach is further highlighted by the fact that an estimated quarter of his donors are people who have chipped in US$200 or less. More than 630,000 donors have never donated to a political campaign before, according to campaign statements.

Obama campaign insiders said they had been under pressure to keep their feet on the fuel pedal, with campaign bosses wanting to take no chances with a historic victory in sight. A vast database of 9 million names is tapped daily with appeals, offers of 'limited edition' magnets and bumper stickers and personally addressed letters from the candidate himself. 'We can't afford to sit on the sidelines,' one of the latest campaign e-mails reads.

'The message is ... just keep going, we're nearly there,' said one activist in Virginia, a Republican stronghold the Obama team is seeking to take.

With much of those donations already spent, both campaigns estimate that Senator Obama is entering the last days with about US$100 million more cash on hand than Senator McCain.

That means more spending on a television and radio blitzkrieg in key battleground states, and more offices and staff on the ground.

In the increasingly important state of Virginia, for example, Senator Obama has opened nearly three times more than Senator McCain's 19 campaign offices.

But in assessing Senator Obama's machine, there is no easy direct comparison with his rival.

Senator McCain, a respected campaign finance reformer in the US Congress, decided to opt for federal campaign grants. This limits the amount he can take from the public for direct campaigning, and must rely on the grants and indirect funds routed through the Republican National Committee.

He had a written agreement during the primary campaign that Senator Obama would join him in accepting federal limits. Buoyed by an impressive war chest built up during his long nomination fight against his Democrat rival Hillary Rodham Clinton, Senator Obama decided to opt out and forge ahead alone.

Sources close to Mr McCain, a veteran senator known for his stubborn stands on principle, stressed he was outraged at Senator Obama's change of heart.

'There was a clear loss of respect at that point,' one well-placed McCain source said. 'We could see his anger.'

Some McCain aides are starting to crank up the rhetoric over finances, suggesting Senator Obama is effectively 'buying the election'. They are also urging him to match Senator McCain's donor transparency and reveal all donors under US$200 - a considerable logistical task.

McCain campaign manager Rick Davis offered 'congratulations' to Senator Obama for his September record, but added: 'Shame on them for not letting the public also see where these contributions are coming from.'

Growing lead

How likely voters said they would cast their ballots in a Reuters/C-SPAN/Zogby poll released yesterday

Barack Obama: 52%

John McCain: 40%