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By Anthony Boadle
BRASILIA, April 29 (Reuters) - Nagging concerns about rising consumer prices and a political scandal involving Brazil's state-run oil company are taking a toll on President Dilma Rousseff's popularity, increasing the chances that an opposition challenger can force this year's election to a runoff, a new poll showed on Tuesday.
The survey by local polling firm MDA also showed that Rousseff's rivals in the October election are finally starting to gain traction with voters and that the race may end up being tighter than many initially thought, though the president remains the clear favorite to win a second four-year term.
If the election were held today, 37 percent of Brazilians surveyed said they would vote for the left-leaning Rousseff, down from 43.7 percent in February. Other recent polls have also showed Rousseff's popularity eroding, sparking a brief rally in Brazilian financial markets as investors gained hope that a more business-friendly challenger could win the election.
Her main rival, Aecio Neves of the centrist Brazilian Social Democracy Party, rose to 21.6 percent support in the new poll from 17 percent in February. Eduardo Campos, the candidate of the Brazilian Socialist Party, also advanced, to 11.8 percent from 9.9 percent.
"For the first time we see strong gains by the opposition," said MDA pollster Marcelo Costa Souza.
Rousseff's support has fallen due to the mushrooming scandal involving oil company Petrobras, concerns over rising inflation and expectations that health and education services will deteriorate and that crime will increase.
The poll showed that seven in 10 Brazilians believe inflation, which is running close to the government's target ceiling of 6.5 percent, will continue to accelerate this year.
It also showed that, among Brazilians who are familiar with Petrobras, nine in 10 want a congressional inquiry to investigate allegations of over-pricing and corruption and eight in 10 believe there were irregularities in the purchase of a refinery in Texas. Two-thirds believe Rousseff, who served as chairwoman of Petrobras' board before becoming president, is partly responsible for the company's troubles.
Brazil's Supreme Court last week ruled that Congress can go ahead with an investigation into Petrobras for the controversial purchase of the Pasadena refinery near Houston and other cases of alleged corruption.
The inquiry is due to be launched next week and could cast further doubt on Rousseff's re-election chances, a scenario that has pushed up the Sao Paulo stock market as investors hope for an end to what they describe as her interventionist economic policies.
The Rousseff administration's approval rating fell to 32.9 percent in April from 36.4 percent in February, the MDA poll said, while the president's personal approval rating dropped to 47.9 percent from 55 percent.
"People are disenchanted with the government. They are starting to change positions, and the campaign has not even started yet," said Jose Agripino of the DEM opposition party. He said the resurgence of inflation, widespread corruption and unfulfilled promises were undermining Rousseff's popularity.
Rousseff's declining numbers have given rise to calls for her predecessor and mentor, Luiz Inacio Lula da Silva, to step in to seek a third term instead of her, his hand-picked heir.
The "Come back Lula" movement gained force on Monday when the small Republic Party, a member of the government's ruling coalition, called on Lula to run again, saying Brazil's rough economic situation required an "experienced and polished political leader" at the helm.
Lula's office said he has no intention of jumping into the race and backs Rousseff for a second term.
MDA surveyed 2,002 eligible voters from April 21-15, and the poll has a margin of error of 2.2 percentage points. (Additional reporting by Maria Carolina Marcello; Editing by Todd Benson and James Dalgleish)
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