Tech-savvy candidates around the country are beginning to court voters online using behavioral targeting technology -- an effective but controversial advertising tool that has raised privacy concerns and drew congressional attention this summer.

Sites using the technology put a small file known as a cookie onto every visitor's Web browser. With cookies aboard, the campaigns can follow users around the Internet, recording what Web pages they visit and getting an idea of the issues they care about.

Businesses have been using online behavioral targeting for years, but only recently have political candidates started to realize its potential. The Obama and McCain campaigns are employing targeting tools, and there are signs the technology is spreading to downballot races. CampaignGrid, a Pennsylvania-based online ad agency, is helping nine Republican House candidates use behavioral targeting, said CEO Jeff Dittus.

If you go to Rep. Jim Gerlach's Web site, for example, and read about his stance on taxes, the next time you browse a related story online -- like a Wall Street Journal article on tax policy -- the Pennsylvania Republican can target you with a banner ad touting his support for tax cuts.

While targeted ads are sometimes more expensive than traditional banner ads, they can attract far more clicks, said Thomas Gensemer, managing partner of Blue State Digital, a technology firm running behavioral targeting programs for the Obama campaign.

"The effectiveness of banner ads is low and decreasing, and it's really about targeting people more precisely," Gensemer said. "It's where campaign budgets are going to expand in the future."

But behavioral targeting technology has raised concerns among privacy advocates, who say Web sites are not being upfront with the visitors they monitor.

"Consumers don't even realize it's happening," said Alissa Cooper, chief computer scientist at the Center for Democracy and Technology, a privacy advocacy group. "A lot of this is about transparency and consumer education and making sure people understand."

Plenty of Web users are uncomfortable with behavioral targeting, according to a poll released Thursday from the Consumer Reports National Research Center. Fifty-three percent of respondents said they were troubled by the idea that companies are tracking their movements online to help show them more relevant ads.

Behavioral targeting has caught the eye of Congress, as well. Senate and House committees held hearings in July to investigate privacy concerns surrounding the technology, and lawmakers have signaled their desire to pass electronic privacy legislation.

Several campaigns using behavioral targeting technology said privacy concerns and the recent public scrutiny are forcing them to tread carefully.

"We don't want to make the mistakes that the private sector had already made," said Mark Campbell, political director for Gerlach's re-election effort. "Political campaigns are concerned about the facts, but also the perceptions."

The Gerlach campaign updated the privacy policy on its Web site last week to provide more details about its use of cookies, Campbell added, in response to inquiries from NationalJournal.com. Among those companies learning the hard way is Google, which recently moved its privacy policy to its home page after withering criticism from electronic privacy advocates and the American Civil Liberties Union that the information wasn't easily accessible.

Maintaining a less-than-robust privacy statement is a dangerous game to play, warned Jeff Storan, director of product management for Platform-A, AOL's digital advertising business.

"From a best-practices perspective, a site owes its users an explanation of what they are doing with that data," he said.

Among CampaignGrid's other clients is Rep. Fred Upton, R-Mich. -- no stranger to the Web himself. The congressman is a member and former chairman of the House Subcommittee on Telecommunications and the Internet, which held the July 17 hearing to investigate the same behavioral targeting technology that Upton may begin employing on his Web site.

Liz Garey, Upton's campaign director, said that while she is unfamiliar with the July hearing, the campaign's awareness of new Web applications "was heightened" because of the congressman's history on the subcommittee.

"We were very interested in stepping up into the more technological areas and getting Fred's name out there," Garey said of the camp's decision to explore behavioral tracking software for future use. "We're excited about the opportunity."