Dark money is a bipartisan weapon. And with both parties locked in a campaign funding arms race, dark money groups are multiplying — and thriving — at both ends of the political spectrum. But, during the 2012 election cycle, conservative dark money groups that reported expenditures to the FEC outspent liberal ones by about 8 to 1, according to the Center for Responsive Politics.

Dark money’s political role was enabled by the Supreme Court’s Citizens United v. FEC decision, which allowed corporations, including certain types of nonprofits, to fund ads expressly urging the election or defeat of federal candidates. That decision overturned previous restrictions on corporate spending to influence election outcomes.

Campaign finance reform activists argue that voters should know the identity of donors behind political advertising in order to evaluate its merits and discern which special interests have a stake in an election’s outcome.

Fred Wertheimer, the founder and president of Democracy 21, has said that “history makes clear that unlimited contributions and secret money are a formula for corruption.” And in a portion of the controversial Citizens United decision, eight of the nine Supreme Court justices agreed that disclosure of money in politics was important because “transparency enables the electorate to make informed decisions and give proper weight to different speakers and messages.”

But supporters of anonymity in politics counter that the Federalist Papers and Thomas Paine’s “Common Sense” were published anonymously during the country’s founding. Lawyers at the Wyoming Liberty Group have argued that throughout American history, “anonymous political speech has been the scorn of entrenched powers and the saving balm of emerging voices.” And the Center for Competitive Politics has argued that “disclosure comes with a cost,” including the potential to chill speech and for donors to be harassed.

The three Republicans and three Democrats who comprise the nation’s primary regulator of campaign funding, the FEC, are divided over the degree to which dark money is a problem. At the same time, the Internal Revenue Service regulates nonprofit organizations and can revoke a nonprofit’s tax-exempt status if a group is deemed undeserving. But in recent years, only a handful of 501(c)(4) nonprofits — social welfare groups by law — have had their tax-exempt status revoked for being too political. The Department of Justice can also criminally prosecute knowing and willful violations of campaign finance law, although this, too, is rare.

Although journalists have developed strategies to track — usually long after the fact — contributions by some nonprofits and labor unions to dark money groups, if an individual donates to a dark money group, there is essentially no public paper trail to follow.

This story is from the Center for Public Integrity, a nonprofit, nonpartisan investigative media organization in Washington, D.C. Read more of its investigations on the influence of money in politics or follow it on Twitter.