Google, rocked back on its heels by a chorus of criticism of its cooperative advertising deal with Yahoo, has mounted a public relations offensive by launching a web site that defends the deal and portrays rival Microsoft as the puppet master behind the “political attacks.”

The moves come amid signs that the U.S. Department of Justice plans to mount anti-trust challenges to the arrangement under which Yahoo’s in-house search advertising system would be supplemented by Google’s AdSense system.

Critics charge that Google, with 63 percent of the search market in August, according to comScore, already dominates search advertising. Adding Yahoo’s 19.6 percent share, they say, would effectively blunt any challenge to Google’s preeminence.

The site, posted by Karen Wickre, Google’s senior manager, corporate communications, at http://www.google.com/yahoogooglefacts/index.html, asserts that: the non-exclusive deal will strengthen Yahoo; ad prices will be set by competitive auction, and provide Yahoo with access to revenue from the Internet’s “long tail,” niche advertising markets covered by the broad reach of AdSense.

Among those opposing the advertising tie-up, which Google and Yahoo plan to launch by mid-October, is the Association of National Advertisers, which contends the deal would stifle competition.

Google, however, likens the arrangement to a deal it already has with IAC’s Ask.com search engine and argues that the alliance enhances competition.

“This agreement - unlike Microsoft's proposed acquisition of Yahoo--means that Yahoo will remain an independent company in the business of search and advertising,” the Google site says. “Yahoo has stated that it will reinvest the additional revenue from this agreement into improving its user services and competing vigorously against Google, Microsoft and other companies.”

The site adds: “The agreement won't affect Yahoo's natural search results. Yahoo will continue to operate its own search engine, and Google's share of search traffic will not increase. In addition, the agreement is non-exclusive, meaning Yahoo could make a similar deal with another company.”

Yahoo’s board of directors and  Chief Executive and co-founder Jerry Yang turned down the takeover offer by Microsoft CEO Steve Ballmer as insufficient before turning to Google, whose search deal is estimated to provide $800 million in incremental revenue.  

Under the agreement, Yahoo has the option to display Google ads alongside its own search results in the U.S. and Canada, but will continue to operate its search engine and in-house advertising systems.

Earlier this week, Yahoo announced APT, a display advertising platform designed to streamline the process of buying and selling online display advertising. While Google dominates text search advertising, Yahoo is the leader in the more fragmented display advertising market.

Two publishers, The San Jose Mercury News and The San Francisco Chronicle, have joined the network, which is expected to be countered by similar rollouts by Microsoft, Google and others.