By Anthony Boadle
BRASILIA (Reuters) - Brazilian President Dilma Rousseff has extended her lead ahead of Sunday's election and would win re-election in a likely second-round runoff, while her main challengers are running neck-and-neck for second place, polls showed on Thursday.
Environmentalist Marina Silva has continued to slip and is now only 3 percentage points ahead of centrist candidate Aecio Neves, according to the Datafolha polling firm, a statistical tie because it is within the poll's margin of error.
If no candidate wins an outright majority in first-round voting, the presidential election will be decided in a runoff between the two leading candidates on Oct. 26.
Both Datafolha and a second poll by the Ibope research firm show Rousseff winning re-election in the runoff by 7 percentage points.
Rousseff increased her lead in the first round to 16 percentage points in both polls, with 40 percent of voter support to 24 percent for Silva.
Neves, the market favorite who has been stuck in third place since Silva's late entry into the race, increased his support by 1 percentage point to 21 percent, Datafolha said, and now has a fighting chance of making the runoff.
The polls confirmed that Silva, a popular anti-establishment figure, has continued to lose ground since peaking in late August. Silva surged after entering the race when her party's original candidate was killed in a plane crash, and initially gained a 10-point advantage over Rousseff.
She lost ground under a barrage of criticism from Rousseff's campaign that portrayed her pro-market policies as a threat to Brazil's poor and questioned her ability to govern Brazil without a solid party coalition.
Rousseff's increased chances of winning re-election have weighed down Brazil's markets where investors are hoping for a change of government. Some blame Rousseff's interventionist policies for the stagnation of Latin America's largest economy.
Silva, a renowned conservationist who has embraced market-friendly policies, is promising to jumpstart Brazil's stagnant economy, while Rousseff vows to extend social programs that have lifted millions of Brazilians out of poverty and reduced inequality in Latin America's largest nation.
The seven main candidates in the race were to face each other in a final television debate late on Thursday, the last day of campaigning.
In her last television ad, Silva assailed Rousseff for lying about the government's role in a bribery and kickback scandal rocking state-run oil company Petroleo Brasileiro SA.
The Datafolha poll surveyed 12,022 respondents nationwide between Oct. 1 and Oct. 2 and the Ibope surveyed 3,010 respondents nationwide between Sept. 27 and Oct. 2. Both polls have a margin of error of plus or minus 2 percentage points.
(Reporting by Anthony Boadle; Editing by Tom Brown)