ProPublica also reported on how other search engine giants are selling their users' browser history to campaigns.

What companies, who follow a model of self regulation, and campaigns are doing isn't popular with the public.

When asked if they wanted "political advertising tailored to your interests," 86% of Americans surveyed said they did not, according to a study from the University of Pennsylvania's Annenberg school of Communication released last July.

Sixty-four percent said their support for a candidate would decrease if they found out a candidate was microtargeting them differently than their neighbor. The study also found that 20% more respondents reacted more strongly to political targeting than they did to being targeted as a consumer.

"There's a lot to say in favor of campaigns targeting voters in this way, (but) there is a lot to be concerned about," said Eitan Hersh, a political science professor at Yale University who studies the impact of microtargeting on campaigns and the political process.

"People like being targeted in many ways," said Hersh. "Many people like that Amazon knows what kind of books they like. If a campaign knows that you're of this religion and this race and went to college, you're likely to have a different set of values ... the campaign is likely to reach out to you on those attributes."

But there is always a danger that the campaign will misfire or that the ads will seem like "pandering," Hersh said.

"The downside, of course, Is that we might not like being stereotyped," he explained.

Still, microtargeting makes an uphill process easier for the campaigns, especially at the presidential level.

"How do you start by trying to convince 200 million people that you they should vote for you?" asked Hersh.

"The task is hard. Data helps and permits campaigns to talk to people about issues they care about," he continued.

And the data does help.

Besides ads that show up before a YouTube video or banner ads on the websites users visit, they dictate scripts that door-to-door canvassers read. They also improve efficiency of campaign voter turnout efforts and reduce costs since ads online are significantly less expensive than television spots.

In the age of DVR, microtargeting can also guarantee that voters actually see campaign ads.

"We can serve a pre-roll video ad," Walsh said, referring to ads viewers see before videos online, "which is great stuff, it forces you to watch it before you get to your content. The big problem for advertisers these days is that everyone is fast forwarding through their videos."

A sign of how often these ads are used by political campaigns -- "online video inventory has been sold out," Lieberman said, in many of the key battleground states looking into the final days of the campaign.

Privacy and civil liberties activists don't propose shutting down online advertising. Instead, they favor an opt-in versus the opt-out option currently available to consumers and voters -- a "Do Not Track" mechanism.

Calabrese says the trick is not getting browsers to add the mechanism, but getting other Web companies to agree to it. Yahoo recently said it would not honor the "Do Not Track" button Microsoft is installing in Microsoft Explorer 10.

"The pushback has been that there is a business model out there that wants to track you all the time," Calabrese said. "They can wring more and more advertising dollars out of you."

But don't look for this practice to end. Lieberman says it's just the beginning.

Looking toward the next election cycle, CampaignGrid signed a deal with AT&T combining AT&T's mobile network with its online voter data files.

The next frontier is to reach voters with ads on their "IP-addressable" television sets, serving the same targeted ads that people see online, during the commercial breaks on their favorite shows. And Lieberman says they are adding data crunching power to what he calls "rich data sets."

As Duke political science professor Sunshine Hillygus said, "There's no turning back on microtargeting."