More from this series
The United States is due to have the first billion-dollar election in its history.
The BBC's Steve Evans presents this two-part investigation into election spending done in collaboration with the Centre for Public Integrity in Washington DC.
Part One - 527s
In the first of this series into money and the US presidential election, Steve Evans looks at how often well-funded independent groups, campaigning on specific issues, may influence the result.
The campaigns themselves will spend over a billion dollars but much of this money won't be under the control of the two major parties.
Instead, it will be channelled by independent groups through two tax loopholes known as 527s, and 501c4s.
The sources of this cash can be difficult to trace as they don't have the same funding restrictions as the campaigns, some of them do not need to declare who their donors are.
In theory, the independent organizations are not allowed to be connected to the campaigns.
However, in practice, their funding is likely to have a major impact on the result and can be useful to the candidates as "attack dogs" that can be easily disowned.
In 2004 a 527, Swift Boat Veterans for Truth, ran a highly damaging campaign questioning Democrat candidate John Kerry's Vietnam War record.
So what role will they play this time around?