The closely watched $94 million Colorado Senate race can be seen as a proxy fight between Karl Rove, the co-founder of Crossroads GPS, one of the nation’s first dark money groups, and Tom Steyer, the former hedge fund guru who emerged as an environmental super PAC sugar daddy.

Crossroads GPS sunk more money into negative ads against Udall than it did against any other candidate nationwide — $8.7 million.

And NextGen Climate Action Committee, the super PAC funded largely through more than $55 million of Steyer’s money, also spent more on negative ads against Gardner than any other candidate — $6.8 million.

Except for the support of the Democratic and Republican parties, the two groups spent more than any other outside groups. And they nearly matched or outspent those party committees: The Democratic Senatorial Campaign Committee spent $8.1 million against Gardner while the National Republican Senatorial Campaign Committee spent $6 million against Udall.

Crossroads GPS and NextGen Climate Action are creatures of the post-Citizens United political landscape. Crossroads GPS is a “social welfare” nonprofit organized under section 501(c)4 of the tax code, and is not required to provide details about its donors. It opened its doors in 2010. NextGen Climate Action Committee, on the other hand, is a super PAC that discloses its donors, ones who can contribute unlimited amounts of cash — and those donors may be people in the broadest sense of the term, including corporations.

Simply put: Before the Citizens United decision in 2010, we wouldn’t have seen a Senate race where two outside groups, one of which does not disclose its donors and both of which may accept unlimited contributions, would account for nearly one out of five dollars spent.

Of course other groups weighed in, too. Indeed, the Colorado Senate race saw more dark money — $35 million, or more than a third of the total spending in the race — than any other race in the country.

Among the other dark money groups that joined in the frey was the U.S. Chamber of Commerce, which spent $2.1 million in support of Gardner and $1.6 million against Udall. The League of Conservation Voters, another social welfare nonprofit, spent $1.9 million against Gardner. Ending Spending, a conservative group that focuses on the federal deficit, spent $1.8 million in favor of Gardner and $1.5 million against Udall. The full list can be seen here.

But even groups that disclose their donors don’t always let voters know who is really backing them. Women Supporting Cory Gardner reported spending more than $81,000 opposing Udall. And who donated to Women Supporting Cory Gardner? Well, that would be the Colorado Women’s Alliance. And what is the Colorado Women’s Alliance? A nonprofit group founded in 2011, according to the Colorado Secretary of State’s office. The group’s founder, Debbie Brown, has described it as a counterweight to “liberals whom she says are obsessed with reproductive politics.” As a nonprofit, the group is not required to disclose its donors.