Brazil President Dilma Rousseff narrowly trailed challenger Aecio Neves in latest opinion polls ahead of their October 26 election showdown, a survey showed.

Neves, who upset predictions to eliminate environmentalist and early front-runner Marina Silva in Sunday's first round, was polling 46 percent to 44 for Rousseff, the Ibope poll for Globo television and Estado de Sao Paulo newspaper showed on Thursday.

The difference is within the margin of error of both polls and considered a statistical tie.

The surveys were the first by Brazil's two major polling firms since Sunday's first-round vote in which leftist Rousseff won 41.6 percent of the votes cast to 33.6 percent for Neves.

Excluding undecided voters, spoiled and blank votes, the polls showed Neves wound win the runoff by 51 percent of the valid votes against 49 percent for Rousseff if it was held today.

Ibope poll interviewed 3,010 voters Tuesday through Thursday and has a margin of error of 2 percentage points.

Datafolha polled 2,884 voters on Wednesday and Thursday. Its poll has a margin of error of 2.2 percentage points. It was commissioned by the Folha de S. Paulo newspaper and the Globo media group.

The polls comes as third-place candidate Silva is due to announce who she is going to back in the run-off, Al Jazeera's Gabriel Elizondo, reporting from Sao Paulo, said.

Our correspondent said that Silva's endorsement could prove decisive, noting that she received 22 million of the popular votes.

"This is an election that is way too close to call," he said, and that makes Silva's decision "all that much more important."