By the time Americans head to the polls in November, they will have been exposed to $3bn of ads. Political ads rarely tell the truth and in this year's presidential election campaign, facts have tended to matter less. This is where the mainstream media should step up. But so far, the US media have not shown the appetite or the stomach to get past the rhetoric and get to the truth. In this week's Listening Post, we look at the politics of telling the truth in a heated election campaign.