By Jane O'Brien
BBC News, Washington
The United States will hold its first billion dollar presidential election next year, heightening concerns about the influence of money in American politics.
Candidates in the 2008 race are seeking donations large and small
In spite of attempts to reform finance laws and limit contributions, the 2008 race for the White House is already the costliest campaign in history.
Election watchdogs say that fundraisers are finding a way around the restrictions to collect unprecedented sums of money.
Corporations are banned from making any donations and individuals can only give up to $2,300 to each candidate in any election.
But according to the Center for Responsive Politics, a non-partisan research group that tracks money in US politics, influential industries are still contributing hundreds of thousands of dollars to their preferred candidates, by means of so-called "bundlers".
These are typically well-connected business people or lobbyists who can tap into their extensive networks to collect and package individual donations on behalf of candidates.
"We see in our records a bundle of, say, 20 or 30 contributions coming on the same day to the same person, all from individuals who work at the same firm," says the centre's executive director Sheila Krumholz.
"We have evidence that there is certainly, in some instances, a feeling that the donors themselves feel pressured, or in their words, extorted - that they have to pay to play."
Bundling is a legal activity and candidates do not have to name their bundlers.
Mary Boyle, communications director for Common Cause, a good-government reform group, explains how "bundlers" have become the new political power brokers.
"When you have the head of an oil company who's helping raise $200,000 for you, it's likely that person is going to have some access and influence to you that other members of the public don't get - and that's where you see the influence in policy," she says.
Common Cause is one of many groups campaigning to make all US elections publicly funded to end dependency on private contributions.
Candidates already have the option to accept public funding but many choose not to do so because they don't think it gives them enough money.
It is currently capped at about $20m - far less than the $27m raised by Hillary Clinton, for example, in the second quarter of this year.
Ms Boyle says the system needs to be overhauled to put all candidates on a level playing field and allow them to focus more on policy.
"This campaign should be about ideas and personalities and leadership, but at this point it's just a struggle to raise money," she says.
"A number of reforms have to be made and we're hoping that it happens by 2012, the next presidential election. It's too late for anything for next year."
The internet has made it easier for ordinary members of the public to make small personal donations.
Barack Obama has attracted large numbers of small donations
Most give less than a couple of hundred dollars, but it is a trend that appears to be helping the Democrats more than the Republicans, who traditionally rely on fewer but more affluent donors and industries.
Ms Krumholz says the Democrats' Barack Obama has raised more money online than any other candidate.
"This is unheard of. It's unprecedented in any previous cycle, and even in this cycle where the internet is playing an important role in all the campaigns, Barack Obama stands out," she says.
He has set a new fundraising record for a Democratic candidate, collecting $32.5m in the second quarter of this year.
And because about half his donors have not hit the legal limit of $2,300, there is still plenty of time for them to contribute more if they want to.
This has given Mr Obama the financial credibility needed to become a serious contender.
Power of money
By contrast, one of the early Republican front-runners, John McCain, may be effectively out of the race or forced to choose to accept capped public funding because he has not raised as much as expected.
John McCain's fundraising has not gone as well as he hoped
It is a chicken-and-egg situation, says Professor Clyde Wilcox, a government expert at Georgetown University who studies voter psychology.
"Someone like McCain looks so far behind that there seems little reason to give because he doesn't have much chance of winning at this point. And as a candidate fades in the polls, money fades also," he says.
But the winning candidate is not necessarily the one who starts out with the most money - although by the end of the race they will probably be the best funded.
"There have been a lot of candidates with relatively small amounts of cash who do pretty well in the nomination process," says Mr Wilcox.
"If the candidate has no money, they can't get the message out. But if they have enough money to be heard then they can attract both votes and donors."
Americans have become increasingly cynical about the power of money in politics. But they are also more likely to make a financial contribution themselves when they are most dissatisfied.
That could help explain the rapidly growing number of small donors - one of the biggest trends of this presidential election.
With approval ratings for President George W Bush and Congress at an all-time low, many voters may be already expressing their views by writing a cheque.