The US election campaign, already breaking all records for fundraising and spending, is set to breach the $1bn (£614m) mark this month, making it the costliest by far in American history.

The two candidates had raised a combined total of $961m by the end of September, according to figures released this week. But Barack Obama alone will almost certainly push that over the $1bn mark before the November 4 election.

The total cost of the presidential race could be as much as $2.4bn, according to the Centre for Responsive Politics, a Washington-based organisation that tracks political fundraising.

The group has calculated that the US elections as a whole could cost $5.3bn when combining the candidates' fundraising with the cost of the congressional election campaign and contributions from unions and other outside organisations and groups.

"The election will blow through historic records on a number of counts," said Sheila Krumholz, executive director of the centre.

In 2004 John Kerry and George Bush raised $880.5m in what was the most expensive election to date.

Obama has so much money available that he is easily outspending John McCain in advertising, which is the biggest expenditure.

Evan Tracey, president of the Campaign Media Analysis Group, which monitors spending on advertising, said Obama was close to saturation point. "There is in essence no limit to what Obama can do. He has the lion's share of spending ... he has more money than TV space to buy," Tracey said.

Since September Obama has bought 330,000 adverts in all the battleground states compared with 100,000 by McCain, Tracey said.

During his political career Obama has seen spending power as a key to victory. He raised a staggering $153m in September, bringing his fundraising total to $603m so far. Of this he has spent $470m. In September alone he spent $96.9m, of which $65m went on advertising, $5m on travel, $1.1m on polling, $1.8m on postage, $1m on setting up stages and $1.2m on credit card fees.

He spent $3m on staff wages but this does not begin to reflect the size of his campaign, which has attracted a huge volunteer base. McCain, by contrast, is struggling to attract Republican volunteers and is offering $12 an hour for canvassers in Florida.

While Obama is free to continue with fundraising efforts this month, McCain has been hobbled by having accepted public finance of $84m that prevents him from raising funds separately.

Having spent $37m of that in September, he had only $47m - which in normal times would have been seen as a substantial sum - for the remainder of the campaign.

Outside Obama's campaign team, his biggest backers are the trade unions, who are spending tens of millions on advertising and campaign staff.

The United Steelworkers union, for instance, has 120 full-time staff working on campaigning for Obama, in addition to volunteers.

Gary Hubbard, a spokesman for the union, was yesterday unable to put an immediate figure on union spending but added: "Democracy ain't cheap."

The Obama team likes to portray its fundraising as a populist movement driven mainly by small donors, contributing $200 or less.

But in an analysis of the 800-page file on fundraising and spending submitted by Obama under election law, the Washington Post found that only a quarter of the $600m raised so far had come from small donors.

McCain potentially has access to the $80m-plus the Republican party has in the bank but some state party officials - either because of coolness towards McCain on ideological grounds or because they do not want to waste cash on what they see as a losing campaign - have withheld funds.

Election inflation

Amount raised by all candidates in previous elections

1976 (winner Jimmy Carter)$171m

1980 (Ronald Reagan)$161.9m

1984 (Reagan)$202m

1988 (George Bush)$324.4m

1992 (Bill Clinton)$331.1m

1996 (Clinton)$425.7m

2000 (George W Bush)$528.9m

2004 (Bush)$880.5m