So says a ThinkProgress investigation:

The largest attack campaign against Democrats this fall is being waged by the U.S. Chamber of Commerce, a trade association organized as a 501(c)(6) that can raise and spend unlimited funds without ever disclosing any of its donors. The Chamber has promised to spend an unprecedented 75 million to defeat candidates like Jack Conway, Sen. Barbara Boxer (D-CA), Jerry Brown, Rep. Joe Sestak (D-PA), and Rep. Tom Perriello (D-VA). As of Sept. 15th, the Chamber had aired more than 8,000 ads on behalf of GOP Senate candidates alone, according to a study from the Wesleyan Media Project. The Chamber’s spending has dwarfed every other issue group and most political party candidate committee spending. A ThinkProgress investigation has found that the Chamber funds its political attack campaign out of its general account, which solicits foreign funding. And while the Chamber will likely assert it has internal controls, foreign money is fungible, permitting the Chamber to run its unprecedented attack campaign. According to legal experts consulted by ThinkProgress, the Chamber is likely skirting longstanding campaign finance law that bans the involvement of foreign corporations in American elections.
In recent years, the Chamber has become very aggressive with its fundraising, opening offices abroad and helping to found foreign chapters (known as Business Councils or “AmChams”). While many of these foreign operations include American businesses with interests overseas, the Chamber has also spearheaded an effort to raise money from foreign corporations, including ones controlled by foreign governments. These foreign members of the Chamber send money either directly to the U.S. Chamber of Commerce, or the foreign members fund their local Chamber, which in turn, transfers dues payments back to the Chamber’s H Street office in Washington DC. These funds are commingled to the Chamber’s 501(c)(6) account which is the vehicle for the attack ads