By Stuart Grudgings
BRASILIA (Reuters) - Brazil's ruling party presidential candidate Dilma Rousseff may be forced into a runoff after an opinion poll on Tuesday showed her lead slipped in the wake of an ethics scandal involving a former aide.
The survey by pollster Datafolha showed Rousseff's voter support fell to 46 percent from 49 percent a week ago, leaving her with 51 percent of valid votes -- just above the threshold she needs to win the election in the first round on Sunday.
The poll was the clearest sign yet that corruption allegations that prompted Rousseff's successor as President Luiz Inacio Lula da Silva's chief of staff to resign are eroding the frontrunner's commanding lead and increasing the chances of a runoff vote on October 31.
"A second round is more possible," said David Fleischer, a politics professor at the University of Brasilia. "It depends on the way TV focuses on scandals for the rest of this week."
The main opposition candidate, Jose Serra of the centrist PSDB party, did not benefit directly from the defections from the Rousseff camp, though he would go to a second round if she fails to win a majority of votes on Sunday. Serra's voter support held steady at 28 percent, the poll showed.
Instead, the shift in allegiances bolstered Green Party candidate Marina Silva, a former environment minister who stands little chance of surviving the first round. Her support climbed to 16 percent from 14 percent, Datafolha said.
Rousseff, a career civil servant who at 62 is running for elected office for the first time, has ridden Lula's immense popularity and an economic boom in Latin America's largest economy to a hefty lead in all opinion polls.
She is widely expected to continue Lula's market-friendly policies and has dismissed the need for major structural reforms, which many economists say are needed if Brazil is to maintain its strong growth of recent years.
Tuesday's poll was the first in more than a month to show a real chance that the election could go to a second round, which Rousseff is expected to easily win barring a major scandal that could potentially turn the tide against her.
The Datafolha poll showed Rousseff, who recovered from a bout of cancer in 2009, with 52 percent of the votes in a runoff versus 39 percent for Serra.
The resignation of chief of staff Erenice Guerra, a former aide to Rousseff, over allegations she was involved in a kickback scheme for public projects followed another ethics scandal over alleged illegal accessing of bank records by members of the ruling Workers' Party.
Both cases have received intense media coverage, apparently prompting some higher-income voters to turn away from Rousseff and raising Serra's hopes of scraping into a second round.
"The Datafolha survey shows that these cases especially affected parts of the middle-class," Datafolha's head of polling Alessandro Janoni wrote in Folha de Sao Paulo newspaper, which published the survey.
Rousseff, who would be Brazil's first woman president if elected, can count on huge support in poorer northern areas of the country where millions have benefited from social welfare programs under Lula. She has also made inroads into wealthier southern regions where the main opposition parties are traditionally stronger.
Securing victory in the first round would boost Rousseff's mandate and top her mentor Lula, whose two election wins came in the second round. It would also give her more time to prepare her government and choose her ministers ahead of the inauguration ceremony in Brasilia on January 1.
Datafolha polled 3,180 people on Monday in 202 cities. The poll had a margin of error of 2 percentage points.
(Editing by Todd Benson and Anthony Boadle)
