A $2.5bn election: Where the money comes from
When the US presidential election is over estimates say it will end up costing $2.5bn (£1.5bn).
That works out to about $20 (£12) per vote.
The money comes from a variety of sources: from the candidates' own fund raising, to money collected by groups called Super PACs.
Super PACs cannot work directly with the Barack Obama and Mitt Romney campaigns, but they spend millions on TV ads hoping to influence the electorate.
All of this adds up to the most expensive presidential campaign in US history.
The BBC will be providing full online live results of the US presidential election on 6 November. More details here
Sources: Federal Election Commission, Center for Responsive Politics, Kantar Media/CMAG
Additional images: Getty Images and ThinkStock