He supports repealing the tax break for oil companies and he's tried to paint McCain as a candidate who is "in the pocket" of big oil corporations.

CNN: Energy compromise offers test for Obama, McCain

During his initial campaign for president, then-candidate Obama decried the unfairness that oil companies were making record profits in tough economic times.

FORBES: Understanding Subsidies

Of the supermajor oil companies, Royal Dutch Shell could be the most likely candidate.

FORBES: Decades More To Drill In 90-Year-Old Permian Oilfields

The head of the truculent oil-workers' union, Carlos Romero Deschamps, is a PRI candidate for the Senate.

ECONOMIST: Mexico��s presidential election

In 2008 the country became a candidate of the Extractive Industries Transparency Initiative - an international project meant to promote openness about government oil revenues - but failed to qualify by an April 2010 deadline.

BBC: Equatorial Guinea profile