The 2014 US mid-term election is projected to be the most expensive ever with a cost of around $3.7 billion (£2.3bn).

Most of the money comes from the candidates themselves, but a significant portion comes from outside groups - some of which don't have to disclose their donors' identities.

In the final days of the campaign season tens of millions of dollars have been spent - especially in key races like those for Senate in North Carolina and Kentucky.

So where exactly does this money come from and what is it paying for? The BBC takes a look and breaks down the numbers.

Produced by the BBC's David Botti