When she launched her party earlier this year, Silva said: "It is not a party created just for the elections. It is a party that calls for a new vision of the world, in which we will be participants, not just spectators."
Recent opinion polls suggest that Silva, who served as environment minister under Rousseff's predecessor, Luiz Inacio Lula da Silva, had emerged stronger from the mass nationwide protests that swept the country in June and caused the popularity ratings of several top politicians to plummet.
A poll conducted in early August by Datafolha, one of Brazil's top polling and statistic firms, said she was the preferred candidate of 26 percent of the country's register voters. Rousseff received the support of 35 percent. Datafolha questioned 2,615 people on Aug. 7-9. The margin of error was plus or minus two percentage points.
Silva won international accolades for her efforts to help preserve Brazil's Amazon rainforest during her tenure as environment minister.
She split with Rousseff's Workers Party in 2009 over policy differences and joined the Green Party. She ran for president in 2010 and won a surprising 20 million first-round votes, but it wasn't enough to get on the second-round ballot.