MEXICO CITY (Reuters) - Mexico’s presidential candidates have declared spending 159 million pesos ($7.8 million) so far on web advertising including Facebook, YouTube and online media, more than a quarter of their campaign spend, the National Electoral Institute (INE) said.

Just six years ago, when President Enrique Pena Nieto was elected, the candidates dedicated less than 5 percent of their money to promote themselves online, INE data shows.

Internet campaigning has been a thorny issue for elections around the world since U.S. investigators found Russia-based groups used social media to try to interfere with the country’s 2016 election. Mexican regulators will continue scrutinizing the spending after the July 1 vote.

Presidential candidate Andres Manuel Lopez Obrador, who holds a double-digit lead in polls, has spent the least of the four contenders on online propaganda at just 3.6 million pesos, INE said in a report published late Wednesday.

That compared with more than 75 million pesos spent by second-place rival Ricardo Anaya and 67 million pesos by ruling Institutional Revolutionary Party (PRI) candidate Jose Antonio Meade.

Lopez Obrador’s campaign says the lower spend reflects a focus on generating content that is then distributed by volunteers, rather than paying for advertising.

“What we have are teams that generate information. The content that we have is what they share, but we don’t pay to promote it,” campaign chief Tatiana Clouthier said earlier this month.

Juan Pablo Espinosa, who runs part of Lopez Obrador’s online campaign, said his team manages a network of 400,000 “amplifiers” on various WhatsApp, Facebook and Twitter groups, which share content with their followers.

Lopez Obrador has also called on his backers, whom the campaign calls “AMLOvers,” to combat critics on social media.

In online monitoring, regulator INE found 709 adverts, more than half of which were on Facebook, with 140 favoring Lopez Obrador, though it was unclear how representative this sample was. The origin of online ads will not be known until after the July 1 vote, when INE will also publish more spending details.

Online advertising is the second largest outlay for the campaigns after physical displays like billboards.

Overall, left-right coalition candidate Anaya has spent the most so far in the campaign, around 259 million pesos. Lopez Obrador’s total spending is less than one quarter of that, 61 million pesos.

Mexico has a mixed public-private campaign finance system, but more than 90 percent of the presidential candidates’ combined income is from public money, the report showed.

Civil society watch-dogs have warned that campaigns underreport spending across the board. For every one peso declared, 15 pesos more goes unreported, nonprofit Mexicanos Contra la Corruption estimated earlier this year.