WASHINGTON -- Every time Nella Stevens logs on to her computer, ads for President Barack Obama or Mitt Romney follow her from site to site, dogging her every digital step as if begging for her vote.

“If I go to one site to research or I start Googling his views on things, then for the next day Obama just scrolls across my screen, and the same thing for Romney,” said Stevens, 52, of Charlotte, N.C. “I started noticing it, and it’s very funny after a while. I was like, ‘This is very strange.’ ”

Stevens is an undecided registered voter in a swing state. Both campaigns would love to find that one issue or argument that will finally get to her, and they’re hoping that data collected from her Web browser and mobile devices can help.

Political campaigns are taking advantage of the same data-mining and consumer-profiling techniques that retailers commonly use to customize ads for certain audiences.

“You can say, ‘I want to reach a registered Democrat who has voted in three of the last four elections with an age 30-50, with an income of $50,000-plus and target ads only to people who are registered and exactly in that population,” said Jim Walsh, a senior partner at DSPolitical, an online advertising network that works with Democratic candidates and liberal organizations.

Ad agencies and data management firms say this "microtargeting" gives campaigns more bang for their bucks, and ensures that they reach the right people with messages tailored to their interests. But privacy advocates say they’re alarmed.

"Powerful interests across the political divide are compiling comprehensive digital dossiers on individual citizens," said Jeffrey Chester, the executive director of the nonprofit Center for Digital Democracy in Washington. "If this was the FBI or national security agency, there would be a huge uproar."

Chester worries that politicians who get elected using such technologies will be reluctant to press for greater scrutiny and regulation of data brokers, the companies that compile personal information about consumers and sell it to marketers – or to campaigns.

“This is the way that marketing is done in the 21st century,” Chester said. “ . . . Campaigns have to use it, but that doesn’t mean there shouldn’t be any rules.”

This year, online election advertising is projected to top $160 million, eight times more than the $20 million spent in 2008, according to the Interactive Advertising Bureau, a trade group of media and technology companies.

Presidential candidates have spent at least $75 million on Internet media in this election cycle, according to the Center for Responsive Politics, a nonpartisan watchdog group.

Romney’s and Obama’s campaigns pledge to protect voters’ private information.

“We do not provide any personal information to outside entities and we stipulate that third party partners not use data collected on the site for other purposes,” Obama campaign spokesman Adam Fetcher said in an email.

“We are committed to ensuring that all of our voter outreach is governed by the highest ethical standards,” Romney spokesman Ryan Williams said in an email.

Walsh’s company, DSPolitical, is just one of many start-ups that sprang up over the past few years to help campaigns deliver finely honed digital ads. Targeted Victory and CampaignGrid, leading firms that work with Republicans, didn’t respond to requests for interviews.