The planned advertising partnership between Google (GOOG) and Yahoo (YHOO), which was devised during Microsoft’s (MSFT) unsolicited bid for Yahoo, is headed for a federal antitrust challenge. And that could mean, according to one analyst, that Google could wind up walking away from the deal.

Two days after the Association of National Advertisers sent a letter to the Justice Department opposing the Google-Yahoo ad pact, antitrust regulators hired high-powered attorney Sanford Litvack to lead its legal challenge to block the deal, according to The Wall Street Journal. For a look at what veteran antitrust lawyer Stephen Axinn told CNNMoney.com about Litvack’s hiring and what it means for Google and Yahoo, click here.

Part of Google’s strategy to form a search ad partnership was to keep Yahoo out of Microsoft’s hands. After failing to strike a deal, Microsoft and Yahoo went separate ways and Yahoo continued to pursue the ad partnership with Google.

Now, it might make more sense for Google to withdraw the partnership plan rather than fight the Justice Department in court, said Stifel Nicolaus analyst Blair Levin. Even though Google and Yahoo don’t need regulatory approval for their ad arrangement, Levin wrote in a research note Tuesday that “it would be risky…to proceed if they are getting signals that the agency has serious concerns.”

In addition, another analyst suggested that Google would not suffer too much if its Yahoo search ad plans were killed.  Cowen analyst Jim Friedland wrote in a note that he thought a Yahoo deal would only boost Google’s earnings before charges by 1% to 2% in the first 12 months of the deal.

Representatives for Google and Yahoo did not immediately return calls seeking comment.

But Google has already voluntarily delayed the start of the joint advertising process until October so regulators could examine its potential impact. “We are confident that the arrangement is beneficial to competition,” Google said in statement Tuesday.

The search ad partnership was first proposed in June when Microsoft went public with its offer to acquire Yahoo. The ad arrangement called for Google to run its text ads next to Yahoo’s search results. In exchange, Google would pay Yahoo an unspecified cut of the search revenue. But from the beginning, the deal between the top two Internet search services invited antitrust scrutiny and, as it turned out, some industry opposition.

After reviewing the deal, the ANA said in its letter to the Justice Department that Google and Yahoo would control 90% of the search ad market. “The partnership will likely diminish competition, increase concentration of market power, limit choices currently available and potentially raise prices to advertisers for high quality, affordable search advertising,” the ANA wrote.

Google responded indirectly to the ANA letter saying that “While there has been a lot of speculation about this agreement’s potential impact on advertisers or ad prices, we think it would be premature for regulators to halt the agreement before we implement it and everyone can judge the actual impact.”

In somewhat related news, Google, in an attempt to ease concerns among regulators, announced on its official blog late Monday that it has decided to shorten the length of time it keeps users’ Web information to 9 months from a previous target of 18 months. Google says it compiles some user information like Internet addresses and search history to better match ads to user interests.