By Anthony Boadle
BRASILIA (Reuters) - Brazil's President Dilma Rousseff has narrowed environmentalist Marina Silva's lead in a likely presidential election runoff in October, a new opinion poll showed on Tuesday, but a new political corruption scandal could hurt her re-election bid.
Silva would win the runoff with 45.5 percent of voter support against 42.7 percent for Rousseff, a gap that has halved from six percentage points in the previous survey by polling firm MDA and lies within the poll's margin of error.
The survey was begun before a scandal involving alleged bribery at state-run oil company Petroleo Brasileiro SA was reported over the weekend. The allegations by a jailed former Petrobras executive that dozens of politicians from the ruling coalition received kickbacks are expected to complicate Rousseff's bid for re-election.
The Rousseff administration's approval rating, however, has risen to 37.5 percent in the poll from 33.1 percent two weeks ago.
The MDA poll showed Rousseff winning the first round of voting on Oct. 5 with 38.1 percent, up from 34.2 percent in the previous poll. Silva remains in second place with 33.5 percent, up from 28.2 percent in the earlier poll.
Support for market favorite Aecio Neves in the first round fell to 14.7 percent from 16 percent two weeks ago.
Silva, a popular anti-establishment figure, surged in the polls after she was thrust into the race last month by the death in a plane crash of her party's original candidate. She is still well-positioned to win the election if it goes as expected to a second-round runoff on Oct. 26 between the top two vote-getters.
Support for Silva appears to have plateaued and she is now facing criticism for flip-floping on issues such as gay marriage and nuclear energy, for which she withdrew her backing.
"Marina has hit her ceiling and she is now a target, while the ruling Workers' Party has plenty of resources to apply to the campaign," said André Cesar, an independent analyst in Brasilia. "Rousseff is showing that she is still competitive and can win this election," he said.
Analysts are divided over the impact of the Petrobras scandal. Some say it will worsen public anger over corruption in Rousseff's Workers' Party, while others expect the allegations will lose steam.
On the economic front, Brazil has tipped into recession this year, but the government has managed to keep unemployment low and layoffs have not affected its voter base.
But in more bad news for Rousseff, Moody's Investors Service raised a red flag on Tuesday saying that it may cut Brazil's credit rating in the next couple of years as the economy slows down.
The MDA poll commissioned by the transport industry lobby CNT surveyed 2,002 people between Sept. 5-7 and has a margin of error of plus or minus 2.2 percentage points.
(Editing by W Simon)