Some of the country's biggest marketers are rallying to oppose an advertising deal between Google Inc. and Yahoo Inc., as the Justice Department considers whether to go to court to block the agreement. The Association of National Advertisers, a trade group that represents major companies like Procter & Gamble Co. and General Motors Corp., sent a letter to the Justice Department Thursday calling the deal bad for advertisers and recommending that it be blocked. The group announced the letter on its Web site on Sunday. The agreement, announced in June, gives Web-search giant Google the right to sell search and other text ads on Yahoo sites, sharing the revenue with Yahoo. Whether the letter will influence federal antitrust regulators remains unclear, but it is considered a blow to Yahoo and Google because of the trade group's high profile. Until now, big marketers have been reluctant to come out against the deal publicly because of Google's growing power in the ad business. Some large advertising agencies and midsize advertisers have endorsed the deal, however, saying they think it will make advertising on Yahoo more effective.