The U.S, part of the Google (NSDQ: GOOG)-DoubleClick approval equation adds another chapter Thursday when execs from Google and Microsoft (NSDQ: MSFT) are supposed to appear before a U.S. Senate panel chaired by U.S. Sen. Herbert Kohl, D-Wis. The case is being reviewed by the FTC, the European Union and regulators in Australia at last count. Google’s David Drummond is slated to defend the acquisition. His argument, according to CNET: “DoubleClick is to Google what FedEx or UPS is to Amazon.com (NSDQ: AMZN). Our current business involves primarily the selling of text-based ads—books in our analogy. By contrast, DoubleClick’s business at its core is to deliver and report on display ads.”

But Microsoft, which closed its acquisition of online ad company aQuantive last month, will argue to the contrary, that Google will aggregate too much power in a core area. CNET (NSDQ: CNET) links to a paper from the AEI-Brookings Joint Center for Regulatory Studies claiming the math shows the concentration would be above the federal warning level.