But Bolsonaro is still leading leftist candidate Fernando Haddad, according to the latest Datafolha poll published by Globo TV, which showed Bolsonaro with 59% of voter support, compared with 41% for Haddad.
In a Datafolha poll last week, Bolsonaro had 58% versus 42% for Haddad.
The poll, which interviewed 9,137 people and had a margin of error of 2 percentage points, suggests Bolsonaro is increasingly likely to win the October 28 run-off vote and become Brazil’s next president.
But a hiccup emerged when newspaper Folha de S.Paulo reported that well-heeled Bolsonaro supporters had paid for messaging by third-party agencies, splashing out up to 12mn reais ($3.26mn) each to spread tens of thousands of attack ads.
The Folha accusation energised Bolsonaro’s struggling opponent Haddad of the Worker’s Party (PT), who said his party has witnesses saying Bolsonaro asked business leaders at a dinner in Sao Paulo for funds to pay for the bulk messaging, which he described as undeclared campaign contributions.
Bolsonaro, who has barely campaigned in recent weeks as he recovered from a near-fatal stab wound, used a Facebook Live video to deny the allegation.
He had earlier described the messaging as “voluntary support.” “Since September 6, I’ve been out of the fight. I was in the hospital 23 days, and I’ve been at home a few days. I did not dine or lunch with anyone. I only went out five times,” Bolsonaro said.
Campaign propaganda has flooded social media in Brazil ahead of the run-off vote.
The issue of fake news and abuse of social media has been a growing concern in elections across the world, and tech giants have come under growing pressure to stem the growth of misinformation.
The Workers Party filed a formal complaint with Brazil’s election tribunal, asking it to investigate the Bolsonaro campaign and five companies cited in the Folha report for alleged abuse of economic resources and illicit use of communication media.
PT lawyers also asked prosecutors and Brazil federal police to conduct a probe.
In a radio interview, Haddad said the Bolsonaro campaign had “created a veritable criminal organisation with businessmen who are using undeclared money to pay for false messages on WhatsApp.” He also called on international election observers and WhatsApp itself to look into the case.
Illegal, under-the-table funding by companies could, if proven, lead to the disqualification of the Bolsonaro ticket, said attorney Guilherme Salles Gonçalves, an electoral law expert.
However, a Brazilian public prosecutor with experience in cybercrime was sceptical whether the allegations would constitute a serious crime.
Legal precedents that apply to public social networks such as Facebook and Twitter may not apply to WhatsApp, where users communicate in smaller groups, said the prosecutor, who requested anonymity to discuss the case.
