Battle for the Senate: Who else is spending like the Koch Brothers?

The big spenders cross the political spectrum from Charles and David Koch and the US Chamber of Commerce on the right to billionaires Michael Bloomberg and Tom Steyer on the left. But the key name in US campaign funding is 'anonymous.'

It’s not just the Koch Brothers. In key Senate-race states across the US, the airwaves are chock-full of political ads – and most of the money is coming from outside groups, not from the candidates’ official campaigns.

This rise of outside groups since 2010 spans both ends of the political spectrum, although the conservative Koch Brothers have come to symbolize the trend.

What’s behind all the outside money? Wealthy donors face federal caps on how much they can give directly to a candidate – but no limit on how much they can put toward political ads created by independent groups.

Here are some of the big spenders as of late September, drawing on information compiled by the Center for Public Integrity and by OpenSecrets.org, the website of the Center for Responsive Politics.

Michael Bloomberg. In his life after being New York City mayor, the billionaire is putting lots of cash behind social-issue politics, from abortion rights to tighter controls on firearms. His support for Democrats comes partly through the group Women Vote, where his $2 million contribution stands alongside smaller amounts from many women and men and groups like labor unions. Women Vote has put $1.7 million so far into TV ads for Democrats in Georgia (where Michelle Nunn (D) is seeking an open seat) and North Carolina (where Sen. Kay Hagan (D) hopes to retain her seat) – two key states in the GOP drive to take back the Senate.

Tom Steyer. Another billionaire with a political cause, Mr. Steyer is putting his hedge fund earnings to work for action on climate change. The Californian has put millions into the Nextgen Climate Action Committee, a group that’s already spent $4.3 million on ads for at-risk Democrats in Michigan, Colorado, and North Carolina – and for Bruce Braley’s bid for an open US Senate seat in Iowa.

Jerrold Perenchio. A businessman known for building the Univision Spanish-language TV empire, he is one of the major funders of American Crossroads. Fueled by Mr. Perenchio and many others, that group in turn has spent $2.8 million so far to support Republican candidates such as Dan Sullivan in Alaska and Joni Ernst in Iowa. If the Crossroads name sounds familiar, that’s because conservative strategy guru Karl Rove is a founder and a symbol of outside-money influence in the 2012 campaign cycle.

Anonymous. Some of the biggest money is also “dark money,” not easily traceable to particular people or groups who have donated it. The Kentucky Opportunity Coalition, for example, sees an opportunity to help reelect Sen. Mitch McConnell, the Senate minority leader. The group has spent $4.3 million in that effort so far this year without having to disclose donors.

US Chamber of Commerce. This major business group, led by Tom Donahue, has poured $5.5 million into Republican efforts in eight states. The group is hoping that a Congress under Republican control will help nudge President Obama forward on priorities such as tax reform and trade agreements.

American Wind Energy Association. Here’s a business group backing Democrats, notably with some $371,000 for Sen. Mark Udall of Colorado. The regulatory and financial breezes will be friendlier for this industry if Senator Udall, who sits on the Energy and Natural Resources Committee, is reelected.

Lily Eskelsen García. OK, she isn’t a top donor personally, but she heads the National Education Association, a major labor union. The union’s advocacy fund has pumped $1.8 million so far behind Democrats in North Carolina, Alaska, and Arkansas. Labor unions, more broadly, are also big funders of other outside groups that are spending for Democrats, including Patriot Majority and Vote Vets Action Fund.

Charles and David Koch. Last but not least, these brothers and their energy-oriented company, Koch Industries, undergird a mammoth network of conservative groups. Americans for Prosperity and Freedom Partners Action Fund, with prominent support from the Kochs, together have spent more than $10 million so far on key Senate races.

Along with all the outside money, it’s important to remember that “inside” money still matters a lot. Incumbent senators have their own big campaign funds, and Senate majority leader Harry Reid (D) of Nevada has already helped to guide more than $9 million into vital races through the Democratic Senatorial Campaign Committee. [Editor's note: The original version of this paragraph gave the wrong name and dollar figure for the Democratic Party committee​ for Senate campaigns.]

All the numbers are still preliminary, with five weeks to go.

So far, outside money has accounted for the majority of airwave spending in North Carolina, Michigan, Colorado, Kentucky, Arkansas, and Iowa, according to tracking as of late September by the Center for Public Integrity.

Polls show a close overall battle, with Republicans within reach of the six-seat gain they’d need to control the Senate.