"Our subcommittee is concerned about competition issues in these markets because of the potentially far-reaching consequences for consumers and advertisers, and our concern about dampening the innovation we have come to expect from a competitive high-tech industry," Kohl said in a statement. Senate Judiciary Antitrust Subcommittee ranking member Orrin Hatch, R-Utah, said he did not see "any immediate yellow flags" from an antitrust front. Competitive Enterprise Institute argued regulators "can best serve consumer interests by leaving well enough alone."

But some watchdogs disagree. They say the relationship could be an initial step toward complete integration of the two firms, raising questions about the collection and sharing of consumer data. The Center for Digital Democracy's Jeff Chester said he will ask antitrust officials in the United States and Europe to "closely and skeptically" examine the deal. "What we are now witnessing is the emergence of a global digital advertising duopoly: Google and Microsoft/Yahoo," Chester said.