The first-round result defied most polling, which had predicted Lula would enter the second round from first place, about three percentage points ahead. Instead, Bolsonaro emerged with a narrow lead, raising questions about what pollsters may have missed in the final days of the campaign.
Political scientist Carlos Pereira of the Getulio Vargas Foundation said part of the explanation lies in a late shift among voters who had intended to support smaller right-wing candidates. Bolsonaro himself had told reporters he expected a "protest vote" from people united in their opposition to Lula, even if they initially backed another candidate.
"Usually, voters vote sincerely in the first round, and strategically in the second round," Pereira said. This time, some supporters of candidates such as former Goias governor Ronaldo Caiado and businessman Renan Santos appear to have made a strategic decision earlier, choosing to back Bolsonaro outright to try to resolve the election immediately.
Analyst Rafael Favetti told Reuters that this "front-loading" of support could actually limit Bolsonaro's ability to expand his coalition further ahead of the runoff, since votes he would typically pick up only in the second round have already been counted.
Another factor was the Banco Master scandal, a sprawling alleged banking fraud that emerged in the final weeks. The case touched figures across the political spectrum, including Supreme Court Justice Alexandre de Moraes, who oversaw the case that sent Bolsonaro's father, former President Jair Bolsonaro, to prison. The scandal injected chaos into the campaign and may have influenced undecided voters.