Flávio Bolsonaro opens the runoff campaign with a narrow lead in Brazil's first major poll.

Datafolha, released Thursday evening for Folha de S.Paulo and Globo, put the senator at 49% and President Luiz Inácio Lula da Silva at 45% in total voting intention. In valid votes — the metric Brazil's election court uses — Bolsonaro led 52% to 48%. The institute called the four-point gap a technical tie at the limit of its two-point margin of error, but said Bolsonaro is more likely to be ahead.

Interviewers spoke with 2,520 voters on Oct. 6 and 7, right after Sunday's first round. Only 1% were undecided. Another 5% said they would cast a blank or null ballot. Ninety-four percent said their minds were made up.

The poll lands two and a half weeks before the Oct. 25 runoff. In the first round, Bolsonaro took 47.03% of valid votes to Lula's 45.16%, a gap of about 2.2 million ballots. Datafolha said 46% of voters who backed eliminated candidates now lean to Bolsonaro, versus 32% to Lula.

Lula still leads among poorer and less-educated voters and in the Northeast. Bolsonaro leads among evangelicals and in the South and Southeast. Government approval remains split, with 49% disapproving and 48% approving.

Bolsonaro has picked up formal backing from allied parties since the first round. Lula said Wednesday that his first-round campaign had been "the most disorganized," Folha reported, and his camp is racing to cut abstention, which hit 21% on Sunday.

How to follow: Brazil's Superior Electoral Court posts official results and poll registrations. The free TV campaign period for the runoff begins Friday.

What we know: Datafolha's first runoff survey shows Bolsonaro narrowly ahead inside a tight margin.
What we don't: Whether later polls will widen or erase that edge before Oct. 25.