Both campaigns are using increasingly sophisticated data-mining techniques, a tactic that is raising questions about transparency.
A few weeks ago, Thomas Goddard, a community college student in Santa Clara, Calif., and a supporter of President Obama, clicked on mittromney.com to check out the candidate's position on abortion.
Then, as he visited other websites, he started seeing advertisements asking him to donate to Mitt Romney's campaign. One mentioned family values, he said, and seemed aimed at someone with more conservative leanings.
"It doesn't make any sense," Goddard said. "I'm the opposite of a Romney supporter. But ever since I went to the Romney site, they've been following me."
One of the hallmarks of this campaign is the use of increasingly sophisticated -- but not always accurate -- data-mining techniques to customize ads for voters based on the digital trails they leave as they visit Internet sites. It is a practice pioneered by online retailers who work with third-party information resellers to create detailed portraits of consumers, all the better to show them relevant marketing pitches. Goddard, for example, may have received those Romney ads because of "retargeting" software designed to show people ads for certain sites or products they have previously viewed.
Now, in the election's final weeks, both campaigns have drastically increased their use of such third-party surveillance engines, said Evidon, a company that helps businesses and consumers monitor and control third-party tracking software.
In September, Evidon identified 76 tracking programs on barackobama.com -- two more trackers than it found on Best Buy's website -- compared with 53 in May. It found 40 trackers on mittromney.com last month, compared with 25 in May.
The report provides a rare glimpse into the number of third-party tracking programs that are operating on the campaign websites -- as many as or more than on some of the most popular retailers' sites.
The campaigns hire some companies, such as ad agencies or data management firms, that marry information collected about voters on a campaign site with data about them from other sources. But these entities, in turn, may bring their own software partners to the sites to perform data-mining activities like tracking the links they share with their social networks.
Some consumer advocates say the proliferation of these trackers raises the risk that data about millions of people's political beliefs could spread to dozens of business-to-business companies whose names many voters have never even heard. There is growing concern that the campaigns or third-party trackers may later use that voter data for purposes the public never imagined, like excluding someone from a job offer.
"Is the data going to be sold to marketers or shared with other campaigns?" said Christopher Calabrese, the legislative counsel for privacy-related issues at the American Civil Liberties Union. "We simply don't know how this information is going to be used in the future."
Adam Fetcher, an Obama spokesman, said the website does not allow its partners to share data collected from visitors with other clients or use it for other purposes like marketing consumer goods. And Ryan Williams, a Romney spokesman, said the campaign abides by the "highest ethical standards."
Industry executives say the campaigns simply use data-mining to show the most relevant message to each voter. "Political campaigns now for the first time can actually reach out to prospective voters with messaging that addresses each person's specific interests and causes," said the Interactive Advertising Bureau.
But privacy advocates say such personalization raises questions about transparency. Calabrese said, "Individual voters may not be aware that the message they are getting is based on information that has been gleaned about their activities around the Web and is precisely targeted to them."