"Google and DoubleClick would have one of the largest search query databases with one of the largest online user behavioral profile databases," 12 Congress members wrote in a letter to Bobby Rush, chairman of the House Subcommittee on Commerce, Trade and Consumer Protection. "The privacy implications of such a merger are enormous and without an in-depth examination, we and the American public will not fully understand what all of those implications may be."

The move comes just days after the FTC concluded a two-day town hall meeting to address how online advertising affects people's privacy. Behavioral targeting has come under particular scrutiny, with advocates charging that consumers don't realize that some ad-serving companies are tracking their behavior across a variety of sites and serving them ads based on their surfing history.

Google's proposed merger with DoubleClick especially troubles advocates because of the prospect of combining consumers' search queries with their surfing history to build detailed profiles.

"Google is an information colossus already, but add on DoubleClick's marketing power and you produce a single commercial entity that can know more about you and me than nearly everybody but mom and the IRS," Barton said in a statement.

Google said in a statement that the company has "taken a number of industry-leading steps to improve privacy for our users, and the success of the DoubleClick acquisition depends on our retaining our users' trust."

"Congress would be best served by taking an industry-wide look at the issue, just as the FTC did at last week's town hall," the company added.

The Center for Democracy & Technology and other groups last week urged the FTC to create a do-not-track list that consumers could sign up for to avoid all behavioral targeting efforts. While many large ad networks allow consumers to opt out of behavioral targeting, not all do. In addition, advocates question whether companies are providing consumers with adequate explanations about behavioral targeting and how to opt out.

The online ad industry opposes that effort, arguing that behavioral targeting in itself doesn't hurt consumers and that government regulation would stifle the growing Internet ad market.

Meanwhile, the Washington research group American Antitrust Institute Tuesday called on the government to prevent the merger. "The upshot of the merger may be higher costs for web publishers to sell their advertising space, which ultimately may affect the diversity and richness of content available on the Internet and the vibrancy of the media," a report issued by the group states.

While this activity was unfolding in the U.S., one of the regulatory authorities in Brazil Tuesday recommended clearing the merger.

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