The announcement, which the Yahoo shareholder called "another shot across Ballmer's bow," irritated Microsoft.
The two companies have deployed swarms of lobbyists to plead their case with lawmakers. Herb Kohl (D-Wis.), head of the U.S. Senate antitrust subcommittee, said Wednesday that he would closely monitor Yahoo's deal with Google to ensure it did not harm competition.
Legal experts say a broader search pact with Google would face intense antitrust scrutiny because of Google's dominance in Web search. If Microsoft raises a formal objection, the review could take months.
Yahoo and Google cautioned that the test did not necessarily mean that they would strike a long-term deal.
But Yahoo appears resigned to outsourcing at least some of its search advertising to Google, which is much more adept at turning searches into ad dollars. Investors have been urging Yahoo to do so for years.
Before Microsoft's Jan. 31 offer, Yahoo had begun exploring outsourcing its Web search advertising in Europe to Google. After the offer, Google CEO Eric Schmidt called Yahoo counterpart Jerry Yang to offer his help in thwarting the Microsoft bid.
"I think it's the first useful move Yahoo has made to put some pressure on Microsoft," activist shareholder Eric Jackson said.
According to people familiar with Yahoo's thinking, the company is ready to raise the pressure even more, exploring alliances with Google designed to withstand an antitrust challenge.
Those include outsourcing a limited number of valuable search terms, such as cars or loans, and outsourcing all search terms to the highest bidder, said the people, who asked not to be named because the talks were confidential.
The latter approach would probably lead to Google's winning most of the advertising, because it has the most efficient process.
"That would generally not be viewed by economists as anti-competitive," UCLA business and law professor John de Figueiredo said.
Times staff writer Meg James contributed to this report.