But some antitrust experts say the planned partnership does raise concerns. Whether this pact is completed or not, they add, it points to the kind of antitrust issues that will increasingly surround Google as a dominant company in the Internet economy, which can quickly magnify the market power of corporate winners.  “Up to now, Google has been very careful to avoid predatory behavior,” said Christine A. Varney, a partner at the law firm Hogan & Hartson and a former member of the Federal Trade Commission. “But a transaction like this, I think, is fundamentally anticompetitive.”  Google argues that cooperative arrangements between companies that also compete are increasingly common. There is even a term for them: co-opetition.  In its antitrust guidelines for partnerships among competitors, issued in 2000, the Justice Department and Federal Trade Commission said: “In order to compete in modern markets, competitors sometimes need to collaborate. ... Such collaborations are not only benign but procompetitive.”  These collaborative deals tend to be easily approved by American antitrust authorities, and Google presented the Yahoo plan as such a deal in response to initial inquiries by the Justice Department.  “There is nothing in antitrust law that says a company can’t use the best technology,” said one person involved in confidential discussions of the deal. “It can’t be bad for competition to help a company become more efficient.”  An antitrust review, legal experts say, would weigh the efficiencies the companies gained through the collaboration against the risk of reduced competition as a result. And antitrust officials, they add, would be less interested in how Google might structure such a deal than its likely impact on the marketplace.  One issue, they say, would be whether a Google partnership would reduce the economic incentives for Yahoo to invest in its own search ad technology, which the company has spent millions in trying to close the gap with Google. By some estimates, Google makes 60 percent more revenue for each search than Yahoo because of its superior technology and larger market share, which attracts more advertisers to bid in Google’s ad auctions.  In April, Google had 61.6 percent of the search market in the United States, according to comScore, while Yahoo had 20.4 percent and Microsoft 9.1 percent. Indeed, the motivation for Microsoft’s bid for Yahoo was to gain enough market share to become a credible rival to Google in Internet search and advertising.  Microsoft dropped its bid for Yahoo this month, but said last weekend that it had renewed talks. Those talks have focused mainly on Microsoft’s buying Yahoo’s search business, not the entire company, say people briefed on the negotiations.  Yahoo conducted a successful test with Google in April in which Google placed ads on some Yahoo searches in the United States. Microsoft is interested in heading off any expansion of the partnership.  In search advertising, market share is important to attract advertisers and Web publishers, but also to continually hone the ad auction technology. The more searches run through a company’s network, the more it learns to further improve its results. If Yahoo hands a sizable portion of its search ad business to Google, analysts say, it loses that knowledge, falling further behind.  So while a deal might be good for Yahoo’s financial performance, at least in the short term, it could reduce competition.  “In this business, where scale is so important, there is a danger that you effectively lose a second major player and the opportunity for a genuine counterweight to Google,” said C. Scott Hemphill, an associate professor at the Columbia Law School.  The anticompetitive impact, in theory, would be higher prices for ads, reduced choice for Web publishers and less innovation in search and search ad technology. “But Google will argue that efficiencies of this kind of collaboration outweigh any anticompetitive risks,” Mr. Hemphill said. “It can claim that it is supplying superior picks and shovels to the entire industry, the best mining tools for gold prospectors in the Internet economy.”  A version of this article appears in print on , on Page C3 of the New York edition with the headline: Google Says It Will Defend Competitive Rationale of a Yahoo Deal. Order Reprints| Today's Paper|Subscribe
How can Google, the Internet search and advertising giant, contend that teaming up with Yahoo in search ads would not invite an antitrust challenge?