The midterm elections are less than a week away, and money is pouring into contested states and districts at a furious pace. A new analysis from Public Citizen shows the biggest “dark money” spender is none other than the US Chamber of Commerce, a mega-trade group representing all sorts of corporations—and one that is spending exclusively to defeat Democrats in the general election.

The Chamber is a 501(c)(6) tax-exempt organization, meaning it doesn’t have to disclose its donors. We know from looking at its board, available membership lists and tax forms from big corporations that much of the Chamber’s money has generally come from titans in the oil, banking and agriculture industries, among others.

The Chamber is leaving a huge footprint in almost every race it enters. The report shows that, through October 25, the Chamber has spent $31.8 million. The second-largest dark-money spender, Crossroads GPS, spent $23.5 million:

Among the report’s other findings:

• The Chamber is averaging $908,000 per race it enters.

• The Chamber is the biggest dark-money spender in twenty-eight of thirty-five races it entered.

• Of the twelve contested Senate races, the Chamber is the top non-disclosing outside spender in seven of those races, spending an average of $1.7 million per state.

• In the twenty-three House races in which the Chamber has spent over $11.5 million, it is the top spender in all but two of them.

• The Chamber has spent mainly to either support Republicans or attack Democrats. The only money it spent against Republicans came early in the year during GOP primaries to support business-friendly Republican candidates.

Thanks to weak campaign finance laws, however, we will likely never know who exactly is bankrolling this massive presence in the midterm elections. “When large corporations decide they want to get their own candidates into office but they don’t want to be seen doing it, they call the US Chamber,” said Lisa Gilbert, director of Public Citizen’s Congress Watch division. “These politicians then push for anti-environmental, anti-consumer and anti-health policies and priorities that hurt everyday Americans.”