BRASILIA, Oct. 4 (Xinhua) -- Brazilians will go to the polls this Sunday, but the winner of the presidential race won't be decided until a runoff is held later this month, according to a leading pollster.

Marcia Cavallari, the director of polling firm Ibope, told Eldorado radio late Wednesday it's unlikely any of the 13 candidates will secure more than 50 percent of the votes on Sunday, the absolute majority needed to forgo a runoff.

"The possibility exists, but it's very small," said Cavallari.

As the election day nears, voting trends tend to shift, but there would have to be a very sudden shift across the country in favor of one of the hopefuls, a scenario that seems doubtful, she said.

The current front-runner is right-wing populist Jair Bolsonaro of the conservative Social Liberal Party, with 32 percent of voter support, followed by runner-up Fernando Haddad of the left-leaning Workers' Party, with 23 percent.

Supporters for Bolsonaro and Haddad are divided largely along class lines, with "higher-income, better-educated" voters in the country's more industrialized south and southeast regions backing Bolsonaro, and working-class Brazilians in the north supporting Haddad.