The campaigns directly hire some companies, like 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 retargeting voters or tracking the political links they share with their social networks.

Now some consumer advocates say the proliferation of these trackers raises the risk that information 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 based on his or her past political affiliations.

“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 and where it is going to end up.”

Evidon offers a free software program called Ghostery that people can use to identify third-party trackers on the sites they visit. On Oct. 18 the program identified 19 different trackers on the Obama Web site and 12 on the Romney site. A reporter contacted 10 for comment.

Among those who responded, Cassie Piercey, a spokeswoman for ValueClick, whose MediaPlex marketing analytics division was identified as operating on the Obama site by Ghostery, said she could not comment on specific clients and referred a reporter to the company’s privacy policy. The policy says that ValueClick may collect information about users — like their Internet Protocol addresses, Web browsing histories, online purchases and searches — that does not involve identifiable information like their names, and that the company may share that data with its clients and marketing partners.

Adam Berke, the president of AdRoll, an advertising and retargeting company identified by Evidon on the Obama site, said the company did not aggregate user data or share it with other clients.