MILWAUKEE — Just because you own a D.V.R. or watch television online does not mean political commercials are not coming soon to a screen near you.

Mitt Romney’s campaign thinks it has found a way to get its ads in front of the increasing number of voters who are not watching traditional television: Find these people online, and show them the ads there.

Here in Wisconsin, where the Republican primary is Tuesday, carefully targeted potential voters will see two Romney commercials on their Web browsers. One is a positive message hailing the candidate’s economic and business credentials. The other is an attack criticizing Rick Santorum as a Washington insider who compromises his core beliefs.

Both commercials, which have been running on local television stations across the state, have gone unseen by many voters — up to one-third of them, by some estimates.

The Romney campaign and a team of online behavior analysts have spent 18 months trying to fight television advertising’s law of diminishing returns, sifting through data on the browsing habits of tens of millions of computer users as the campaign builds a richly detailed cache of potential supporters.

In doing so, Mr. Romney’s strategists are hoping to turn the Web into a political persuasion tool, signaling a shift in the way modern campaigns view digital advertising. It is no longer merely a supplement for traditional media like television. In some cases, it is a substitute entirely.

A survey conducted last May on voters’ television viewing habits, which is often cited by Romney advisers, found that 31 percent of likely voters had not watched television “live” — that is, at the time it was being broadcast, as opposed to online or on a recording device — in the previous week. And of the 17 percent who said they mostly watched programs recorded on devices like a D.V.R., a large majority skipped through ads most of the time.  The nationwide telephone survey was conducted by Public Opinion Strategies, a Republican polling firm, and SEA Polling and Strategic Design, a Democratic polling firm.

“This will likely become the first truly digital election because so many people are not paying attention to live TV,” said Darrell M. West, director of the Center for Technology Innovation at the Brookings Institution. Saying that many campaigns were beginning to integrate the study of online behavior into their digital strategies, Mr. West said, “In many respects, it’s analogous to the emergence of TV advertising in the 1960s.”

Television will still account for a vast majority of the money spent on political advertising — billions this year, and 9 out of every 10 advertising dollars, strategists estimate. But with the return on that investment becoming less of a sure bet, campaigns are taking advantage of technologies to track and model browsing behavior as never before.

The Obama digital team, for example, knows when supporters have opened an e-mail from the campaign and whether they have clicked on tabs in the e-mail that direct people to BarackObama.com. It uses that data to determine whether to send more or fewer solicitations.

The campaign has also hired a “chief scientist” who worked in the private sector finding ways to discern consumers’ interests from data online, and used that data to target messages to entice people to buy certain products.

Republican efforts in this type of digital strategy get less attention but are just as sophisticated.

Rather than buying ads on specific Web sites, the Romney campaign sees greater value in buying audiences — which remain anonymous, identifiable only by a numeric code — that are built through careful analysis and predictive behavior modeling.

“We are site agnostic and audience specific,” said Zac Moffatt, the Romney campaign’s digital director. “It doesn’t bother me what site they’re on. I’m just looking for an audience.”

Using outside digital strategists, the campaign analyzed surveys from Democratic and Republican pollsters that determined, among other things, how often people were watching live television. Then strategists took that data, paired it with browsing histories and built a model to identify voters who are not likely to be watching live television.

Reaching those people is only one element of the campaign’s effort to reach individual voters online. Though the Internet has always been useful in preaching to the converted — soliciting donations, rounding up volunteers, rallying voters on Election Day — it has been far less effective at coaxing undecided voters.

So the campaign set out to identify potential voters who are most likely politically conservative and might vote for Mr. Romney but need more persuasion. Here in Wisconsin, these people will see Web ads with the positive message about Mr. Romney’s economic leadership, but not the one that mocks Mr. Santorum.

The group the campaign has designated as potentially persuadable was culled from surveying thousands of online users about party affiliation, positions on key political issues and opinions about the president. From those responses, the campaign’s outside digital strategy firm, Targeted Victory, was able to narrow down the type of people it wanted: 18 and older, Republican-leaning and strongly dissatisfied with the current administration.

Using the Web histories of the people who fit that profile, Lotame, an audience analytics company the campaign has hired, uses algorithms to find other computer users who might have similar political sentiments based on their browsing. Looking at what these people do online — what they read, where they leave comments and what content they share with friends — all helps refine the sample.

When the Romney campaign puts its ads in front of these people, it can tell whether they watched the videos, which requires someone to scroll over the ad and hold the mouse there for three seconds, how long they spent watching and whether they took any actions because of the ad, like sharing their e-mail address, giving money or posting content to Twitter or Facebook. Knowing who is responding to the ads helps the campaign refine its ideal audiences even further.

“When I look at the people who click on that banner ad or looked at that video or went to the donation page, we can unpack a whole new set of behavioral variables,” said Adam Lehman, Lotame’s chief operating officer.

Along the way, the Romney campaign has learned a few common characteristics of its online supporters. They tend to like to take online quizzes on news and entertainment Web sites. They like to share photographs. And they are interested in topics like technology, literature, home repair and child care.

But just as important is knowing where a message is likely to fall on deaf ears. In that case, the campaign has discovered certain traits that tend to be associated with people who do not respond to Mr. Romney’s ads. For example, their online behavior shows they are interested in video and casino games, bowling, martial arts and jazz.

These patterns can vary by state. For example, in Michigan, people who visited Christian music Web sites were not fans, something strategists attributed to the likelihood of such people being Rick Santorum supporters.

The results of putting this data to use have been encouraging. Targeted Victory says that people who see audience-specific Romney ads engage with them at a rate three to five times higher than they do with a standard ad.

“As much of the fat that you can clean out, that’s an advantage to the campaign,” said Ryan Meerstein, the senior political analyst at Targeted Victory. “Our system is getting smarter every day as we learn more about these users.”