Campaign-finance operations are mutating at a frightening pace; each successive election since the Supreme Court’s Citizens United decision has birthed new varieties of independent expenditure groups that deliver even more access for wealthy donors and corporations. This year, single-candidate expenditure groups are pervasive and represent perhaps the apex of this evolution.

Single-candidate expenditure groups are Super PACs or nonprofits that spend unlimited money to support just one candidate (or to attack only that candidate’s opponents). They are a descendant of the candidate-specific groups like Priorities USA that formed to support just one candidate for president in 2012—but now most Congressional candidates in tight races have one at their side.

These are extremely attractive options for donors because they offer the ability to evade campaign-finance limits entirely: if you are a coal magnate that wants to give Mitch McConnell money, you can’t donate more than $5,200 to his campaign this cycle under FEC rules. But if you turn to the Kentucky Opportunity Coalition, the 501(c)(4) that is exclusively supporting McConnell, you can give as much as you want. You don’t have to worry about giving money to a national Republican Super PAC that may or may not spend your money on McConnell’s race—you can effectively fund a shadow McConnell campaign in Kentucky.

Moreover, corporations and labor unions that are forbidden from contributing directly to candidates can freely give to these candidate-specific groups. And in the case of the Kentucky Opportunity Coalition and many other similar outfits, the public will never know because those groups are not required to disclose their donors.

“This is the future unfolding before our eyes, and it’s as dangerous as anything that’s going on in money in politics,” said Fred Wertheimer, president of the campaign finance reform group Democracy 21. “It is the end product of a Supreme Court that apparently set out to destroy the nation’s anti-corruption campaign-finance laws.”

A new report from the Brennan Center for Justice lays out how these groups are spending much more than they have in the past, while also disclosing less. Through the end of September, the ten biggest “buddy groups,” as they are sometimes called in the campaign-finance world, have spent $26 million in contested Senate races. Only four of these groups fully disclose their donors; many offer no disclosure at all:

While these groups did exist in 2012, they are far more common for members of Congress in the 2014 midterms. But the truly ominous turn is how many of them now are dark-money outfits that don’t disclose donors. Ian Vandewalker, the author of the Brennan Center report, noted that dark-money single-candidate groups weren’t significant spenders in past Senate races.

Republicans appear to have a monopoly on the biggest of these groups, though the Brennan Center concludes that the Senate Majority PAC, run by former staffers of Senate majority leader Harry Reid, is probably elbowing out candidate-specific PACs for Democrats by serving as a catch-all for endangered Democrats. It is also almost wholly funding Put Alaska First, listed above, which is the second-biggest candidate-specific spending group in the midterms.

In some races, like Colorado, Iowa and Louisiana, buddy groups aren’t very active. In others, like Kentucky and Alaska, they are the biggest non-party spenders in the game, the report shows:

The Brennan Center also says single-candidate spending groups “are a major factor in House races this cycle.” Three of the thirteen toss-up House districts have seen “significant” spending by single-candidate groups, and in two of those races single-candidate groups “dominate” the outside spending.

The heart of the problem is the chance for corruption: big donors are using these groups to supercharge their influence over candidates. The Brennan Center looked at the donors to candidate-specific Super PACs, which must disclose donors, and found seventy-six donors who maxed out to a candidate’s campaign while also donating to a buddy group. For the dark-money groups, it’s reasonable to expect roughly the same pattern.

Looking at buddy groups that accept individual contributions, the Brennan Center found that 99 percent came from double-dipping donors. Only one group had less than 70 percent of donations from such donors.

Federal election law puts donation limits in place to limit potential corruption of candidates by powerful donors, but buddy groups are laying waste to that principle. “It’s as obvious as can be that if someone can give a million dollars, in essence, to directly support a candidate, the million dollars creates the opportunity for quid pro quo,” Wertheimer said.

Moreover, significant corporate and labor union money is no doubt flowing into these groups, which would otherwise be forbidden. Much of the corporate money is hidden; it’s true across the campaign finance world that corporations much prefer 501(c)(4) groups that don’t disclose donors, as opposed to Super PACs.

Even if these buddy groups were operating independently of campaigns, there would be the opportunity for serious corruption. But election law allows these groups to be run by former aides to the candidate. The official campaign and the buddy group are also allowed to share vendors, including for strategy and messaging, and the candidates can raise money for the buddy group. Campaign materials can also be used by buddy groups—that’s why Mitch McConnell put up wordless b-roll of the senator smiling that later became widely mocked.

The Kentucky Opportunity Coalition, to extend the example, is run by Steven Law, a former top McConnell aide. In 2012 The Washington Post said Law and McConnell “remain a powerful combined force in Washington.”

Sometimes single-candidate groups can be a family affair. The Post reported earlier this year about some unusual arrangements in the primary campaigns: for example, Mike Turner, running for a House seat from Oklahoma, had by his side a buddy group funded mainly by his mother and grandfather. Ben Sasse, a Republican who is likely the next senator from Nebraska, was boosted in the primary by a buddy group where the sole funder was his 87-year-old great uncle.

Priorities for Iowa, which is solely backing Republican Senate candidate Joni Ernst, is run by a consulting firm that also employs a consultant for Ernst’s official campaign, the Brennan Center report notes. They claim there’s a “firewall” in place, but many analysts find this common defense laughable.

“This is a charade. Everyone who’s involved knows what’s going on,” said Wertheimer about single-candidate groups generally. “The idea that this is independent from the candidate’s campaign and therefore you don’t have to worry about campaign limits is absurd.”