That is a fear many in the industry had expressed for months.

“Despite the careful construction, the deal does not pass the smell test of the No. 1 and No. 2 search players working directly together,” said Jonathan Miller, the former chief executive of AOL. In 2005, Mr. Miller helped negotiate a search and advertising partnership between AOL and Google.

Mr. Barnett said the decision to abandon the Google-Yahoo deal eliminated the need for government intervention, suggesting that in their investigation, regulators had not found cause for further action against Google.

Still, the Justice Department’s threat to block the deal, legal experts say, was a departure as well as a sign that Google would receive increasingly close scrutiny in the future.

Under Mr. Barnett, antitrust enforcement in the Bush administration has been done with a light hand. Merger deals, for example, have typically been given a green light. And as Google pointed out, its planned tie-up with Yahoo was much less than a merger  a commercial contract for Google to handle some of Yahoo’s search advertising.

Yet officials took a long, hard look at the agreement and found it troublesome.

“For all we say about how fast these high-tech markets change, the really big players  Google in Internet search ads, Microsoft in personal computer software, Intel in microchips  do seem to stay big,” said Herbert J. Hovenkamp, a law professor at the University of Iowa. “A market in which Google and Yahoo compete against each other in Internet search advertising looked a lot better to Justice than one in which they cooperate. That’s got to be the subtext here.”

It is not clear how the Justice Department under a new administration would view Google’s growing power. Democrats generally take a more aggressive stance than Republicans on antitrust issues. But the incoming administration may not be inclined to go after Google, whose chief executive, Eric E. Schmidt, campaigned for Senator Obama.

As Google has extended its power into every corner of the Internet and beyond, a clash with regulators was inevitable. And some analysts said Google actually calculated the risk of such a confrontation before it offered the ad partnership to Yahoo.