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 almost tied 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.
Rousseff has advanced to within 3 percentage points of an outright victory in the first-round voting, or 47 percent, when spoilt and blank ballots are excluded, Datafolha said.
If no candidate wins a majority, the election will be decided in a runoff between the two leading candidates on Oct. 26.
Rousseff's increased chances of winning a second term 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.
Both the Datafolha poll and another by the Ibope research firm show Rousseff winning a runoff against Silva by 7 percentage points.
Rousseff has increased her lead over Silva 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 favourite 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.
In the final television debate of the campaign on Thursday night, Silva and Neves criticized Rousseff for allowing corruption at state-run oil company Petroleo Brasileiro SA (SA:PETR4), which is embroiled in a scandal over alleged kickbacks and the bribery of politicians.
The allegations contained in leaked plea bargain statements by a jailed former director of Petrobras have so far not hurt Rousseff's re-election bid because no evidence has emerged.
But Silva assailed Rousseff for lying about her relationship with the jailed executive.
"Corruption has been swept under the carpet," she said. Neves accused Rousseff's party of "handing the country's largest company over to a gang of thieves."
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 & Kim Coghill)