But the past 48 hours have seen what many describe as a shameless populist pitch from Senator Clinton to seal the vote in Indiana, a state hit hard by the loss of manufacturing jobs.

Last year, Senator Clinton appeared on the cover of Fortune magazine under the heading "Why business loves Hillary" but now she's pitching to blue-collar voters by slamming Wall Street and lamenting how the working class "don't want the game rigged against us".

Yesterday, she inched up the rhetoric further, raising the unlikely possibility of trying to break up the Organisation of Petroleum Exporting Countries.

"That's not a market. That's a monopoly," she said, saying she would use US anti-monopoly laws as well as the World Trade Organisation to take on OPEC.