A new poll ahead of Brazil's presidential election in October shows former president Luiz Inacio Lula da Silva maintaining a significant lead over Jair Bolsonaro, but the right-wing incumbent is gaining ground.

47% of those surveyed by the Datafolha consulting firm said they intended to vote for Lula, the 76-year-old leftist leader, in the first round election on October 2, the same result as last month's poll.

That represents a 15-point lead over the far-right Bolsonaro (32%), which is slightly narrower than the 18-point gap last month, but a significant jump over the 21-point spread in May.

Lula's strong showing in the poll means there is a small chance he could ultimately receive over 50% in the first round, avoiding a runoff against 67-year-old Bolsonaro.

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If the election does advance to a second round, on October 30, the poll shows Lula beating Bolsonaro with 54% of the vote.

The two candidates, running in the most polarized presidential race in decades, are far ahead of any third-party challengers -- although center-left Ciro Gomes received 7% in the poll.

Though the two front-runners have been campaigning for months, the official launch was on Tuesday.

Bolsonaro started with a rally in Juiz de Fora, while Lula, a former union leader, chose a car factory in Sao Bernardo do Campo as his symbolic backdrop.

Datafolha said it interviewed 5,744 people between last Tuesday and this Thursday in 281 Brazilian cities and its results include a margin of error of +/- 2 points.

Read more: Brazilians protest Bolsonaro's threat to democratic institutions