NEW YORK » As they peruse the Internet, voters in New Hampshire and Iowa are probably seeing ads for Mitt Romney and President Barack Obama alongside those for shoes and holiday gifts. The ads will then follow those voters around the Web, popping up on news sites, Google searches and on social networking sites like Facebook.

Online advertising, once used primarily as a way to reach young and heavily wired consumers, has emerged as an essential communications tool in the 2012 presidential contest. While few expect Web ads to supplant television commercials anytime soon, strategists say online ads may be the most nimble, efficient and cost-effective way to reach voters.

"Online advertising cuts through because of its ability to target. It’s unparalleled in any other medium," said Romney’s digital director, Zac Moffatt. "TV may be more effective for driving a big message, but per usage, the Internet is more powerful. We are probably one presidential cycle from everyone believing that."

Web ads can take many forms, from small display boxes to clickable videos to 15- or 30-second commercials known as "pre-rolls" a viewer sees before the start of a news clip or YouTube video.

Targeting is key. While campaigns invest heavily in television ads to reach a mass audience, Web ads are geared specifically to people based on their ZIP code, demographics and, most importantly, their Internet browsing history.

That means someone who has visited the Obama campaign website will likely start seeing his ads on a number of different Web pages. And those who use Google to search for information on the Republican candidates might notice a Romney campaign pre-roll the next time they watch a TV show online.

Campaigns also buy ads on websites that cater to the different demographic groups the campaigns are hoping to reach.

"When someone expresses interest in politics online, it’s an incredibly good time for the campaigns to talk to them," said Andrew Roos, a Google account leader who works with Democratic campaigns on Web ad strategy. "You want to grab people when they are paying attention and ask them to take another action, like send money or attend an offline event. It’s an old-school organization principle that has been working its way online."

Campaigns were slow to adapt to online advertising even as the corporate world flocked to the Web with product ads years ago. Internet ad revenue climbed to nearly $7.9 billion in the third quarter of 2011, up 22 percent from the same time last year, according to the Internet Advertising Bureau, which tracks online ad spending.

Corporations now spend from 18 to 28 percent of their advertising budget online, while campaigns historically have spent no more than 5 percent.

Chris Talbot, a freelance campaign digital strategist, noted that big companies can devote considerable time, money and research to figuring out what works online and what doesn’t. Campaigns don’t have that luxury.

"There is no ‘next quarter’ in politics, so campaigns usually revert to a template of what’s worked in the past," he said.

In 2008, Obama and Republican presidential rival John McCain both did limited online campaign advertising. Web ads grew more prevalent in the 2010 midterm elections, when 85 of the top-spending House races and 600 interest groups bought display ads on Google.

To be sure, plenty of Internet users say they aren’t thrilled with the proliferation of online ads, particularly those that follow them from site to site.

A USA Today/Gallup poll taken in late 2010 found 9 out of 10 respondents said they pay little attention to online ads. Two-thirds said they don’t believe advertisers should be able to target them based on their past Web searches.

"The only way it works is on a mass scale. Most people ignore ads on the Web," said Aaron Shapiro, head of the digital marking firm HUGE.

Web ads’ biggest advantage, many strategists say, is accountability.

"Online ads are very metric driven — you can figure out how many impressions you got, how many people clicked, how many people signed up for an email address. All of that is calculated in real time," Google’s Roos said. "It’s much more efficient than direct mail and TV."

The Romney campaign’s Moffatt said Web ads became part of the media strategy when officials there realized how much their own viewing habits had changed.

"Strategists here acknowledge they really don’t watch live TV," Moffatt said.