Google (NSDQ: GOOG) and Yahoo (NSDQ: YHOO) arrive on Capitol Hill tomorrow, set to defend (or more likely explain) their search ad deal to camera-hungry Senators. Also testifying the hearing (10:30 AM ET) will be representatives from Microsoft (NSDQ: MSFT), Yellowpages.com and Askthebuilder.com (presumably brought in to get the little guy’s view). Google Chief Legal Officer David Drummond will point out, according to a prepared statement seen by MarketWatch, that the agreement does not expand Google’s share of the search market, and that the alternative outcome, Yahoo selling to Microsoft, would be much worse for competition.

And while the two companies look to set the senators at ease, antitrust officials from various states and the DOJ have started sending out subpoenas looking for more info, according to MarketWatch. This is routine, says the report, but it underscores the fact that there are plenty of legal hurdles out there, and the companies could easily get snagged on any one of them. Whether there’s any legitimate basis or not, politicians love any chance they can to take a stand.