Advocacy groups take aim at Google-Yahoo partnership

"If there were a deal (with Yahoo), we would anticipate structuring the deal to address the antitrust concerns that have been widely discussed," said Google Chairman Eric Schmidt.

Although Schmidt wouldn't detail such a structure, analysts have speculated that it would operate the partnership as an auction-style system that would allow other rivals, including Microsoft, to show ads on Yahoo.

The two companies first began discussing a partnership as an alternative to Microsoft's efforts to purchase Yahoo, which fell apart last weekend.

The civil rights groups noted in their letter that Google has about 70 percent of the search advertising market, and Yahoo 20 percent.

Search engines are "the primary portal into the Internet" and direct "hundreds of millions of online users to ... the most relevant information, news, entertainment, education and e-commerce," the groups wrote.

"No single gatekeeper should have unchecked control over critical segments of the market," the letter said, which was also signed by the National Black Chamber of Commerce and several Latino groups.