A group of Northern California Democrats representing Silicon Valley wrote Attorney General Michael Mukasey to say they were "deeply concerned" that the Department of Justice may try to pre-emptively block the Yahoo-Google ad partnership.
In their letter, they emphasized that the partnership, announced in June, was nonexclusive and wouldn't affect Yahoo's search results, which would continue to operate independently.
Calling such a suit against a "nonexclusive" contract unprecedented, they said it could harm the online ad market and electronic commerce.
They added that the suggestion that the partnership would result in Google controlling up to 90% of search-based advertising "is simply not the case."
They were careful not to tell the DOJ what to do, but they said it needed to "carefully assess" the consequences and precedent of challenging the deal.
Google has been pushing hard for the deal, while Microsoft, which has been trying to grow its own search business, warned that it would give Google too much market power.