Candidates pay for adverts to appear in searches for opponent.
Political 'hijacking' on high seas of the web
Seeking to woo online voters, Barack Obama, the US president, and his Republican challenger, Mitt Romney, have adopted an advertising tactic referred to as brand hijacking that companies have used for years to market wares to web users searching for information about competitors.
People who type one candidate's name into Google's search box in some markets have seen adverts for his opponent. A search for "Barack Obama", for instance, has yielded results for Mr Romney, while entering "Mitt Romney" has resulted in adverts for Mr Obama. Mr Romney has used a similar tactic on Facebook.
The approach is designed to help the candidates raise awareness among users of the web or social networks. The practice has also stirred controversy when used by companies, resulting in lawsuits that alleged it enables trademark breaches.
While court rulings have been mixed over whether the form of advertising is legal, it can breed confusion for voters seeking information, said Peter Harvey, a lawyer at Harvey Siskind in San Francisco.
"There are some real negatives in terms of causing consumer confusion and misdirecting people," said Mr Harvey, who practices intellectual property law. "And it's quite intentional. So, it's a problem."
Eric Goldman, a professor at Santa Clara University's law school in California, said some voters may consider it a "bare knuckles" marketing method that is beneath candidates seeking the highest office in the US.
"We might view it as not presidential enough," said Mr Goldman, who added that he does not find it off-putting. "I could see other people saying, 'I don't think that's the way we want the marketing presented to us'."
Buying adverts that appear when people search for a competitor occurs commonly on search engines. Google, for instance, lets advertisers pay for their message to appear as a result alongside those of a search for a competing brand.
Michael Young, an attorney based in Plano, Texas, labelled the practice "brand hijacking".
Mr Obama and Mr Romney have both spent on this form of advertising this year, according to estimates by Rise Interactive, a digital-marketing agency based in Chicago.
The amount spent on these adverts amounted to tens of thousands of dollars or less in the third quarter, according to Rise Interactive. That is pocket change for campaigns with budgets upwards of US$2 billion (Dh7.34bn) combined for this election.
Still, showing up in internet searches is an important tactic, said Zac Moffatt, the digital director for Mitt Romney's campaign.
Facebook's take on this kind of advertising is newer and less proven than Google's. It only began testing a form of the adverts in July. Rather than using keywords like Google, Facebook lets marketers bid on categories that people may be looking for on the social network.
"We think of search a lot of times as intent," Mr Moffatt said. "If you can participate in that conversation, the likelihood that you're relevant is much higher. It's pretty specific."
The Romney campaign's digital staff has more than 140 peopleand has bought query-based adverts called "sponsored results", as well as other forms of advertising, such as marketing messages tailored to mobile users, Mr Moffatt said.
While Mr Obama has more supporters on Facebook, Mr Romney's fan page may be creating more buzz. Mr Obama has attracted 31.7 million "likes", while Mr Romney has 11.9 million. Still, the number of people leaving comments or shared posts about Mr Romney's page was recently at 2.5 million, compared with Mr Obama's 2.34 million.