This could be big. The U.S. Supreme Court has ruled that a conservative political group must disclose donors who pay for its explicitly political ads in the coming midterm elections.

Chief Justice John Roberts had issued an order protecting Crossroads GPS from a lower court ruling, but the full court reversed him.

CREW’s executive director, Noah Bookbinder, said the Supreme Court ruling “is going to affect spending in the 2018 elections. Groups that run these kinds of ads — ads that tell you to vote for or against another candidate — are going to have to disclose their contributors, and that is incredibly important.”

The Supreme Court’s decision comes less than a week after a new research report by the government reform group Issue One, which puts some dollar amounts on what these unreported donors are giving. The report, which took a year of research, finds that the top 15 politically active nonprofits raised and spent more than $600 million on campaigns between 2010, when Citizens United boosted secret fundraising, and 2016.

The secret giving is made possible by a regulatory loophole at the FEC. The groups, usually organized as 501(c)(4) social welfare organizations or 501(c)(6) business associations, don’t register as political committees with the commission. With the loophole, the FEC wants donor disclosure only when a donor earmarks the money for specific ads.

The top four spenders identified by Issue One are the U.S. Chamber of Commerce, the mainstream conservative Crossroads GPS, the Koch network’s Americans for Prosperity and the National Rifle Association. Issue One says that collectively, the four groups pumped at least $357 million into elections between 2010 and 2016.

These big spenders should be familiar. The U.S. Chamber of Commerce is currently beating up Democrat Clarke Tucker, opposing the millionaires’ friend, French HIll. The defendant in this case, Crossroads GPS, and the Koch network have a long list of Arkansas beneficiaries, with Sen. Tom Cotton at the top. Dark money groups were hugely influential in the defeat of Courtney Goodson for chief justice two years ago and dark money is expected to flood in again for David Sterling, her opponent this year. Attorney General Leslie Rutledge will be lavished with dark money.

There is this potential asterisk, however: The key is “explicit’ political advertising. I still think there’s room for non-disclosure of financing of overtly political advertising that doesn’t expressly call for a vote for or against a candidate. Thus: “Candidate Y is a child molester. Call him and tell him you don’t like it.” Or, “Candidate Z believes in motherhood and apple pie. Call and thank him.”

We’ll see.