WASHINGTON — Any two people interested in whether Amanda Beard is dating fellow Olympian Michael Phelps, and who clicked on the Boston Herald tidbit that raced around the Web recently, got the same piece of gossip: “Rumored galpal Amanda Beard on Phelps: No thanks!”

What was different was the political ads that appeared — or didn’t — beside the story.

Readers who had visited Barack Obama’s Web site received as many as three Obama ads alongside the gossip. “Help Elect Barack Obama President of the United States” and “Visit the Barack Obama Website,” the ads said.

Readers who hadn’t visited his site didn’t see a single Obama pitch.

How did the campaign know which readers to send ads to? Although both the Obama and McCain campaigns are reluctant to discuss details, the ability to identify sympathetic voters based on their Internet habits, and then to target them with ads as they move across the Web, is one of the defining aspects of the 2008 presidential campaign.

Getting out the (right) vote

Digital advertising networks and large Web companies such as Yahoo and Microsoft are using Web behavior — which news articles people read, which blogs they visit or what search terms they enter — to target voters who may be sympathetic to a certain cause. Using a method known as “sentiment detection,” some companies even boast that they can tell whether the blog you go to is for or against the Iraq war.

“During a get-out-the-vote drive, you don’t want to get out the wrong vote,” said Diane Rinaldo, political advertising director at Yahoo, which has worked with both campaigns. With these techniques, the candidates “can reach who they want to reach without wasting their incredibly valuable media dollars, and reach them with the right message.”

The advertising techniques, known as “behavioral targeting” and “retargeting,” have raised alarms from some privacy advocates, who say no one should unwittingly have their political leanings analyzed as they use the Web, or be tracked for the delivery of political ads. Congress has begun looking into the use of such techniques for commercial advertisers.

“The Web has been hailed for creating new opportunities for political expression, but there is this dark underside to it,” said Jeffrey Chester with the Center for Digital Democracy. “Yes, you can reach everyone — but you can track, target and profile them as well, and none of this is disclosed.”

Advocates of the practice, which is common in commercial marketing, say its use in the political world is comparable to traditional direct-mail campaign practices. Direct-mail efforts, they note, combine voter registration and other records to identify targets. They then send tailored pitches to their homes.

By contrast, most of the online targeting is directed to a Web browser, and the name and home address of the target is unnecessary.

“Both campaigns are embracing online targeting ad technologies,” said Michael Bassik, vice president of interactive marketing at MSHC Partners, a leading Democratic communications firm. “It sounds scarier, but it’s less intrusive than direct mail ever was.”

Gathering data

Guessing how a person might vote — and whether they might be receptive to a pitch — has long been part of the science of political marketing.

But the Internet creates many new ways for campaigns to gather data about potential voters, and then to reach out to them.

Both presidential campaigns are using “retargeting” to send ads to people who visited their Web sites but who didn’t leave their name or e-mail address.

To track those visitors even after they’ve left, the site places a small file, known as a cookie, on the visitor’s Web browser. When that person visits another site, an advertising system can send a tailored ad after detecting the cookie, which indicates that the person is a potential voter for the given candidate.

That’s how the Obama campaign can send an ad to a person long after they’ve visited the Obama site, even when their mind is on something far afield from politics — like Phelps and Beard.

The cookie might even indicate a user’s interests, allowing the campaign to further tailor an ad. For example, looking at the cookies from John McCain’s site reveals that a person who visits looking for information about gas prices is tagged that way.

Using that information, the campaign could send the user an ad about McCain’s energy policy.

“If you responded to a certain kind of ad, we could hit you with a similarly themed ad at another time,” said Michael Palmer, the eCampaign director for McCain. “Without violating any privacy concerns, we try to know as much about our users as possible.”

Detecting political leanings

Identifying potential supporters is also increasingly easier with the Internet, because what a person reads or browses on the Internet can reveal hir or her political leanings.

Specific Media, a company that has worked with both sides in the presidential race, combines data about users — some of which it buys and some of which it receives from partners — to create profiles on about 175 million people, according to the company’s senior vice president David Jakubowski.

The data it collects includes information about what articles the person has read on some newspaper sites, what blogs and forums the person attends and what other sites are visited.

Using sentiment detection, Specific Media can judge whether a blog about the Iraq war or tax cuts is generally in favor or opposed to those policies. That helps them determine the political leanings of a visitor.

Gathering data on all the Web visits people make, the company can then present a political campaign with “buckets” of voters described as Republican or Democrat, conservative or liberal, and by what specific issues the person — identified by a cookie on their browser — may be interested in.

“You look at the patterns — you take known Democrats and say, ‘How do they behave on the Web?’” Jakubowski said. “One data point doesn’t put you in a data bucket. If you read a lot of U.S. politics, whether it’s blogs or news or opinion, and you tend to read more of the stuff about conservative policies, you will then end up in a more conservative bucket.”

He said that when the company identifies someone’s party affiliation, the information proves to be nearly 100 percent accurate.

Similarly, Yahoo collects information about the 140 million unique monthly visitors to its sites. The company records what kinds of stories a user has read at Yahoo News — one of the most popular news sites, as well as what search terms a person has entered in the company’s search engine.

Yahoo began a year and a half ago, creating sets of Web behaviors that matched any of the potential candidates — even former vice president Al Gore, who never entered the race but was a long-rumored possibility.

The “buckets” that Yahoo offers to candidates indicate a voter’s interests. There are categories for the Iraq war, energy and the economy, and also whether they are “Obama-interested” or “McCain-interested.”

“To see the two presidential campaigns using behavioral targeting is very telling of how powerful a marketing tool it is,” said Mike Zaneis, vice president of policy at the Interactive Advertising Bureau. “There is a growing level of awareness that there is a certain level of tracking going on online. But they may be surprised how prevalent its use is in political campaigns.”