BRASILIA, Aug 26 (Reuters) - 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 percent of voter support heading into the Oct. 5 vote, according to the survey by polling institute Ibope.
The poll showed Rousseff with 34 percent, down from 38 percent in the previous Ibope survey in early August. The other main opposition candidate, Aecio Neves, had 19 percent support, down from 23 percent in the last Ibope poll.
In a likely second-round runoff on Oct. 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 Aug. 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. (Reporting by Anthony Boadle; Editing by Todd Benson and Diane Craft)