The Panama Papers, millions of stolen digital documents leaked to a German newspaper, have exposed how a large, Panama-based law firm, Mossack Fonseca & Co., helped leaders of Russia, Pakistan, and other countries, as well gun-runners, drug dealers, soccer bureaucrats, and private citizens, hide money from taxes and disclosures. The firm says it is cooperating with investigators and drops clients if it finds out they aren't honest.

Tax avoidance by multinational corporations is also a presidential campaign issue.

"This is very unusual in terms of the size and scope. But it also fits in a larger way with what has been going on," says Peter Hardy, a former federal criminal tax prosecutor, now a partner at Ballard Spahr LLP in Philadelphia.

Criminal prosecutions in the late 2000s and new laws have made it harder to hide money in offshore havens. The Panama Papers publicity "will push this further," Hardy says.

"The irony is that while the U.S. has been very aggressive in suggesting other countries become more transparent to the U.S. taxing authorities, the U.S. is finding that it is regarded as a tax haven for foreign investors" who export money here to avoid paying taxes at home, Hardy adds.