Money has been at the centre of US politics for decades. Candidate Barack Obama raised $750mn to help him get elected as president. That went down as the most expensive federal election in history - but this year it is due to get even more expensive.

Millions of dollars are being spent as the race for the US presidency gets underway, and this year corporations and unions are spending record amounts to influence the election.

As a result of a US Supreme Court judgement, known as the "citizens united" ruling, outside groups such as corporations and unions can now raise and spend unlimited money as long as they do not coordinate with the candidate.

The ruling is being challenged in a number of cities. Critics of it say it empowers corporations with their vast treasure chests over individual donors. The Supreme Court says money is a form of free speech, but critics argue that corporations are not people and therefore do not have the same right to free speech.

Another criticism is that candidates are less likely to be held accountable when negative advertisements are funded by outside interest groups on their behalf.

What are the implications of the "citizens united" ruling? And what effect is the increasing amount of money involved having on US politics?

Inside Story: US 2012 discusses with guests: Anne Kornblut, the White House correspondent for the Washington Post; Daniel Newman, the executive director of MapLight, a nonpartisan research organisation that reveals money's influence on politics; and JD Gordon, who has worked on the Herman Cain campaign as the vice-president of communications.