*Rousseff maintains lead ahead of runoff vote
*Only a major scandal likely to stop her winning
*Other recent polls showed her widening lead (Adds details from poll)
By Hugo Bachega
SAO PAULO, Oct 25 (Reuters) - Brazilian ruling party candidate Dilma Rousseff maintained her comfortable lead over opposition challenger Jose Serra ahead of a presidential runoff vote on Sunday, according to an opinion poll.
Rousseff had 49 percent of voter support compared to Serra's 38 percent in the survey by polling firm Vox Populi, which was published on Monday by Website iG. The last Vox Populi poll on Oct. 19 showed Rousseff with 51 backing versus 39 percent for Serra.
The poll follows three surveys last week that showed Rousseff pulling away from Serra as the campaign shifted from social issues and back to the economic gains under her political mentor, President Luiz Inacio Lula da Silva.
That followed a difficult few weeks for Rousseff in which Serra narrowed the gap as he capitalized on doubts among voters over Rousseff's views on abortion and her alleged involvement in scandals.
Rousseff would have a 14-percentage-point lead of 57 percent versus 43 for Serra once blank and void ballots are excluded, as they are on election day, the poll showed.
Last week a survey by Datafolha, Brazil's most respected polling firm, showed Rousseff's lead at 12 percentage points when those ballots were taken out.
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Rousseff's lead suggests that only a surprise such as a major scandal directly linking her to corruption could stop her from becoming the first woman to lead Brazil.
The new poll showed Rousseff with greater support than Serra among religious voters, including evangelical Christians, whose last-minute doubts about Rousseff's religious values were crucial in keeping her from winning outright in the election's first round on Oct. 3.
A scuffle last week in Rio de Janeiro between Serra backers and Rousseff supporters, in which Serra was struck on the head by an object, appeared to have had little effect on the standing of either candidate.
Serra's camp has tried to portray the incident as showing the left-leaning ruling Workers' Party is aggressive and lacks control.
TWO MORE DEBATES
Vox Populi spoke with 3,000 people on Saturday and Sunday for the poll, which has a margin of error of 1.8 percentage points. Six percent of voters plan to submit blank votes, while 7 percent did not respond or said they were undecided.
Polls before the first round of the election on Oct. 3 overestimated Rousseff's lead, failing to fully predict a large swing to third-placed candidate Marina Silva that deprived Rousseff of the majority of votes needed for outright victory.
But analysts say that the dwindling number of undecided voters means there are fewer wild cards in the race that could help Serra, who failed to win the endorsement of Silva's Green Party for the second round.
The former Sao Paulo state governor has two more televised debates, one on Monday and another on Friday, to mount a last-minute attack against Rousseff. But he has failed to make much of a mark in previous debates.
Rousseff, a 62-year-old career civil servant running in her first election, appears to have successfully shifted the campaign back to her strong suit -- the huge economic strides and improved living standards that Brazil has achieved during Lula's eight years in power.
The candidate is expected to broadly continue Lula's mix of market-friendly policies and social programs while expanding the role of the state in some key areas of Latin America's largest economy.
(Reporting by Hugo Bachega, Writing by Stuart Grudgings and Brian Ellsworth; Editing by Stacey Joyce)