Clintons have a history of being secretive, and do not help themselves by withholding otherwise innocuous information so that it becomes loaded with conspiracy-thinking.

In theory, Bill Clinton's global philanthropy business and his wife's unpredictable political campaign are separate. In practice, at least two dozen of Hillary's $100,000 campaign donors, or 'Hillraisers', are also donors to Bill's foundation. According to the New York Times, Bill has raised substantial sums abroad, including funds from the Saudi royal family, the governments of Kuwait and Qatar and the king of Morocco.

But for several weeks late last year, the foundation suggested that a previously undisclosed $31.3m donation had not come from a single donor. Then, shortly before Christmas, Giustra acknowledged the contribution. Meanwhile Hillary has declined to release her tax records, a pro forma step for any politician seeking high office.

US campaign finance laws are clear: an absolute prohibition on foreign donations to presidential campaigns and a limit on US citizens of $2,300 in direct

contributions. Presidential foundations, however, are unconstrained by the sum or source of donations. "The vast scale of these secret fund-raising operations presents enormous opportunities for abuse," noted the California Democratic congressman Henry Waxman earlier this year.

Whether Hillary Clinton's tax records bear fruit for Obama, or Clinton's relationship with Giustra turns out to be more than altruistic, the high-flying, money-minting former president is at least trying to get some of his affairs in order.

In January, Clinton confirmed he was negotiating a large payout before cutting ties with Yucaipa, the investment company owned by his fun-luvin' friend Ron Burkle. Last week, it became clearer why: a team from Brazil's Labour Ministry found "degrading" living conditions for sugarcane workers employed by an ethanol company whose investors include Yucaipa and several of Bill's friends.

A spokesman said Clinton found the allegations "deeply troubling" and reiterated that he was taking steps to ensure there is "an appropriate transition" for his business relationships should his wife win the nomination.