Former Vice President Joe Biden is telegraphing that he plans to attack his rivals on the debate stage for a lack of transparency into their finances. Biden is expected to go after Sen. Elizabeth Warren in particular, Bloomberg reported, for failing to disclose details of private income during the 1990s and 2000s from the types of companies that she now lambasts for “rigging the system.” Warren, as a law professor, did consulting work for private companies that involved her bankruptcy expertise, including advising Dow Corning in 1995, involving a major settlement with women harmed by breast implants. Warren has released her tax returns dating back to 2008.

Biden may be looking to hammer her for hypocrisy, but his charge of a lack of transparency is badly undercut by his own financial opacity — not decades ago, but in the last two years. Since leaving the White House, Biden, long proud of his wealth ranking near the bottom of the U.S. Senate, began delivering high-dollar speeches to well-heeled clients and raked in book revenue that elevated him well into the upper class. He earned some $15.6 million in the last two years alone, according to financial disclosures released by his campaign. It is typical for presidential candidates to voluntarily release their tax returns, with the exception of Donald Trump. Despite those releases, the details of how Biden, whose career has been partially dedicated to enabling financial secrecy in Delaware, made a significant portion of that money remains a mystery. The Bidens have used their home state’s financial privacy laws to shield his income from public view, by setting up two tax- and transparency-avoidance vehicles known as S corporations. He and his wife Jill Biden called them CelticCapri Corp. and Giacoppa Corp., respectively, and, according to the Wall Street Journal, have reported more than $13 million in profits the previous two years that weren’t subject to specific disclosure or self-employment taxes. As CNBC has described, money Biden made from book deals and speeches flowed into the S corporations and was then remitted to Biden and his wife as “distributions” rather than salary. When money is funneled through an S corporation, the recipient doesn’t owe Social Security or Medicare taxes on it, nor can the source of revenue be traced. (In addition to the distributions, the Bidens drew relatively small salaries from the S Corporations: under half a million dollars, for which they owed self-employment taxes.)

Left/Top: Former Speaker of the House Newt Gingrich addresses the press at Trump Tower on Nov. 21, 2016. Right/Bottom: Former Sen. John Edwards speaks in Decorah, Iowa, during his unsuccessful run for the Democratic presidential nomination on Dec. 27, 2007.Photos: Kevin Hagen/Getty Images; John Gress/Reuters

The use of S corporations by politicians is known as the Gingrich-Edwards loophole, named for Newt Gingrich and John Edwards, who have deployed it in the past. The strategy has become popular among Republicans, but Democratic politicians typically avoid it, given its capacity to appear hypocritical. Warren, Barack Obama, Hillary Clinton, and Bernie Sanders, the Journal has reported, all shied away from the strategy when it came to reporting income from books and speeches.