In Mexico Presidential Race, a Bruising Battle Online Marco Ugarte/Associated Press Enrique Peña Nieto of the PRI campaigning in Nezahualcóyotl, Mexico. Limits on political advertising are forcing presidential candidates to adapt. MEXICO CITY - It sounds like the typical hardball, American-style campaign. The presidential candidate from the incumbent's party calls the front-runner a "liar" in television and Internet advertisements. Supporters of the front-runner retaliate with a Web site and Twitter posts that say his top opponent "lies." And the third-place candidate wraps the gaffes of both of them into a YouTube video cheekily titled "Excuses Not to Debate." State-of-the-art, no-holds-barred political warfare, perhaps, except that after President Felipe Calderón narrowly won a divisive race here six years ago that featured ads calling his opponent a danger to the country, Mexico's political establishment had vowed that it would tolerate no more of that. But a law passed in 2007 that was intended to keep campaigning orderly and clean - it bans the Mexican equivalent of political action committees, limits spending, regulates language in advertisements and tightens the official campaign period to just 89 days - has been undercut by the unpredictable and uncontrollable Web. On Web sites and in the online social media, a parallel battlefield has emerged as candidates vie for the support of voters, more than a quarter of whom, polls say, have not made a choice as the July 1 election nears. Many of the undecided are part of the fast-growing bloc of young middle-class Mexicans who tend to be more politically independent and may prove pivotal in determining the country's next president. "If you want to win a campaign you need to win every space of the terrain," said Agustín Torres Ibarrola, a 34-year-old lawmaker who coordinates the digital strategy for Josefina Vázquez Mota, the candidate of Mr. Calderón's National Action Party, or PAN, who trails by double digits in the polls. Mr. Torres was sitting beside a large screen displaying his TweetDeck page, which manages Twitter and Facebook accounts, as a handful of young campaign workers hunched over laptops monitoring social media sites and posting material related to a dispute with the campaign of the front-runner, Enrique Peña Nieto of the Institutional Revolutionary Party, or PRI. Recently, Mr. Torres used his Twitter account to take a veiled swipe at Mr. Peña Nieto, alluding to suspicions that his party, which governed Mexico for seven decades until 2000, would make deals with drug gangs. The election, Mr. Torres wrote, was "about choosing between politicians who fight drug trafficking or politicians who tolerate it." "Which country do you want?" he asked. Just under a third of Mexico's population regularly uses the Internet (compared with 80 percent in the United States). But the campaigns have seen how social media sites can help shape public opinion - newspapers here closely track and publish the number of each candidate's Twitter and Facebook followers - and they skirt the heavily regulated airwaves. Often using automated programs or armies of volunteers, the campaigns battle to land trending topics on Twitter and celebrate them as important discussion points. Last Wednesday, "Josefina gets confused," a reference to a verbal gaffe by Ms. Vázquez Mota, was a popular topic for much of the day. So far, the weighty problems facing Mexico - the drug war, feeble job growth, persistent poverty and the failings of the police and judicial system - have received little attention and generated only vague pronouncements. Instead, the campaigns expand and refine their digital attacks, often using hard-to-trace and easily disavowed volunteers and supporters to do the dirty work. Aurelio Nuño Mayer, the media director of the Peña Nieto campaign, said his operation relied on about 20,000 volunteers to post Twitter messages and drive up the popularity of favored topics. While the volunteers are ordered not to undercut Mr. Peña Nieto's positive message of efficiency - he is broadcasting new ads this week equating the divisiveness in the race this year to that of the 2006 campaign - Mr. Nuño Mayer acknowledged that the campaign could not always control them. "Twitter is like a jungle," he said. With the anonymity, it is like a free-for-all. A dizzy spell by Ms. Vázquez Mota during a speech and her failure to directly answer a student's question on education policy ricocheted across YouTube and Twitter, though none of it carried the signature of her opponents" official campaign or party. Mexico has taken one of the more aggressive approaches toward regulating campaign speech, with the result that parties are repeatedly complaining to the election commission about opponents" ads and remarks, and then calling the decision biased when it goes against them.