Democratic presidential front-runner Hillary Rodham Clinton has made it a point in some early speeches to call for cleaning up the dark money flowing through U.S. politics. "We need to fix our dysfunctional political system and get unaccounted money out of it, once and for all, even if that takes a constitutional amendment," she said in April in Iowa, describing campaign-finance reform as one of the "four big fights" of her campaign.

If she is serious, Clinton might take a closer look at a recent transaction that benefited the main super PAC supporting her campaign, Priorities USA Action. The group, which is theoretically independent of the Clinton campaign but clearly is supporting her and has longtime supporters of Clinton on its board, received a $1 million donation that cannot be traced, according to a report by the Associated Press (AP).

Super PACs are campaign-finance vehicles that are supposed to be independent of campaigns and can accept unlimited amounts of donations, as permitted by the Supreme Court's Citizens United decision. Super PACs are required to identify their donors. But as the trail of the $1 million to Priorities USA Action shows, there are gaping loopholes. In this case, dark money moved to a super PAC without the original donors being disclosed.

Priorities USA Action got the donation June 29 from another super PAC, called Fair Share Action, the AP reported. And that super PAC received the money from two nonprofit "social welfare" groups, Fair Share Inc. and Environment America Inc. These organizations, regulated by the Internal Revenue Service, do not have to publicly disclose their donors. According to the AP, Fair Share advocates for job creation, and Environment America works on issues such as climate change. The groups told the AP that their funds come from many small donors, but the exact origin of the $1 million remains in the shadows. A spokesman for Priorities USA Action insisted that the group is "playing by the rules" and that it is unwilling to "unilaterally disarm" in the presidential fundraising arms race.

This opaque shuffling of cash from one group to another is exactly what Clinton meant by "unaccounted money." She should tell all groups supporting her — including the supposedly "independent" super PAC — that they must identify the original sources of contributions, or else give the money back.

The IRS has been re-examining the use of "social welfare" groups as dark-money channels, but the agency has indicated that whatever rules are forthcoming will not be applied to the 2016 election cycle. This is a mistake; the IRS needs to reduce the amount of hidden political money flowing through these groups — and do it now, not later. Otherwise, the abuses will grow. Dark money is a recipe for corruption. The $1 million gift that made its way to Priorities USA Action suggests a convergence of all that's wrong with the current system: a dangerous mix of secrecy and unlimited cash.

FROM AN EDITORIAL IN THE WASHINGTON POST