Marina Silva narrows Dilma Rousseff's lead, could win Brazil presidential election runoff: Opinion poll

President Dilma Rousseff has a narrow lead over environmentalist Marina Silva in Brazil's presidential race but could lose the election if it goes to a second-round runoff between the two, a new opinion poll showed on Tuesday.

Silva, who was thrust into the presidential race last week following the death of her party's candidate, has 29% of voter support heading into the October 5 vote, according to the survey by polling institute Ibope.

The poll showed Rousseff with 34%, down from 38% in the previous Ibope survey in early August. The other main opposition candidate, Aecio Neves, had 19% support, down from 23% in the last Ibope poll.

In a likely second-round runoff on October 26 between the top two vote-getters, Silva would defeat Rousseff by a margin of nine percentage points, the poll showed.

Ibope surveyed 2,506 people nationwide between August 23-26. The poll published on the website of the O Estado de S. Paulo newspaper has a margin of error of plus or minus 2 percentage points.