Google may be getting cold feet. In a last-ditch effort to avoid a merger with Microsoft, Yahoo said it was considering teaming with Google in a search advertising deal. But some Google executives are now questioning whether that’s a good idea, the Wall Street Journal reported Thursday.

One major hurdle: A Google-Yahoo tieup could face tough scrutiny from regulators in Washington and the European Union. Last month Yahoo (YHOO) ran a two-week test displaying some of Google’s (GOOG) search ads on Yahoo’s homepage. Both Yahoo and Google executives said the experiment went well. The two are reportedly in talks to outsource Google’s search technology in a non-exclusive arrangement.

Spokespeople from both Google and Yahoo declined to comment.

Google may have wedged its way into the mix in order to break up Microsoft (MSFT) and Yahoo. Microsoft CEO Steve Ballmer admitted that Google was a factor when the software giant walked away from its $47.5 billion offer last Saturday. In a letter to Yahoo CEO Jerry Yang, Ballmer said his company would not be willing to deal with the “host of regulatory and legal problems” that it would inevitably inherit if Yahoo partnered with Google.

Last month Microsoft’s general counsel Brad Smith lashed out at the two big Internet sites for partnering even in a limited test. He argued that a Google-Yahoo combo would give Google a 90% share of online search advertising and that “this would make the market far less competitive. It would be fair to say that Microsoft would aggressively lobby against a long-term partnership between Google and Yahoo.

Microsoft was one of Google’s biggest detractors when the search giant said it was going to buy DoubleClick, the top firm in online display advertising. Google got approval from both the Federal Trade Commission and the European Committee to acquire DoubleClick, but the approval took the big G nearly a whole year.

Both the FTC and the European Committee ruled that text-based search advertising and display advertising, which is the preferred way that big brands like to advertise, are two different markets, and therefore the merger was not anticompetitive. But regulators may be more wary if the two biggest players in search want to team up.

“Google has incredible chutzpa,” said Jeffrey Chester, the executive director of the Center for Digital Democracy. The public interest group had opposed Google’s DoubleClick deal because it would give Google an overwhelming lead in online advertising.

Chester said both Microsoft and Google have approached him to support their political message on the Hill. He has not yet endorsed either party, and is waiting for a deal – whether it’s Microsoft and Yahoo or Google and Yahoo – to be announced.

However, Chester said he is wary of a Google-Yahoo tieup. “Whatever happens, we don’t want Google to operate Yahoo out of its back pocket,” Chester said. “Whether or not regulators do something about it, we’ll do something.”