"In order to ensure that American consumers and competitors are given the 21st century safeguards they require, both the DOJ and FTC must carefully examine how the proposed
Microsoft/Yahoo agreement will impact the digital marketplace," they write in the letter, which will also be sent to FTC Chairman Jon Leibowitz and Senate Judiciary Antitrust Subcommittee Chairman Herb Kohl, D-Wis. Kohl previously said his panel would closely review the deal. An ad arrangement between Google and Yahoo fell apart in 2008 after regulators warned they would file a lawsuit to block it.

The letter argues that the proposed combination of Microsoft's and Yahoo's search platforms effectively undermines the latter as a meaningful competitor as it gives up its ability to offer marketers a robust search and display combination. DOJ must ask whether the plan is "simply a precursor to the eventual absorption by Microsoft of Yahoo's various advertising holdings" and whether the combination of their data collection, profiling, and targeting technologies could place competitors at a disadvantage, they state.