By Paulo Trevisani 

BRASÍLIA--Brazilian senators decided Tuesday to open an investigation into allegations that managers of Brazil's energy giant Petróleo Brasileiro SA overpaid for a Texas refinery in 2006.

Such a probe, for which a new poll shows wide public support, comes ahead of general elections in October. President Dilma Rousseff's was chairwoman of Petrobras, as the government-controlled, publicly traded company is known, at the time of the deal and personally approved it. Members of her Worker's Party, or PT, headed the energy company's management at the time.

Current and former officials of Petrobras are being accused by opposition lawmakers of having made a flawed decision to buy the oil refinery in Pasadena, Texas, in 2006. Questions about the deal were first raised in the local press a couple of years ago, and have gained prominence more recently as opposition leaders use the case to pick at Ms. Rousseff's popularity.

Government representatives and a Petrobras spokeswoman declined to comment, but the company has denied wrongdoing and has said it is conducting an internal investigation.

The president's political allies, who make up a majority in congress, have fought the opposition's move to install a parliamentary commission to investigate the issue. But late Tuesday afternoon, Speaker of the Senate Renan Calheiros, whose party, PMDB, is central to the governing coalition, said an investigation will begin in the upper house next week.

Mr. Calheiros didn't have many options after the Supreme Court last week ruled that the minority had the right to request the scrutiny. The opposition is now trying to expand the investigation to include the lower house.

Earlier Tuesday, an opinion poll released by the National Confederation of Transportation, or CNT, and research firm MDA, showed that 30.3% of respondents said they are following the Pasadena case in the news, and of these 91.4% said they wanted Petrobras to be investigated.

Current and former officials at the company have said the deal seemed good at the time, but became less appealing as global oil-market conditions changed in the following years. The deal began with Petrobras agreeing to pay $359 million for 50% of the refinery owned by Belgian commodities trader Transcor Astra Group--which had bought it for $42.5 million a year earlier.

Ms. Rousseff has said she wasn't informed that a put option in the original contract would later force Petrobras to buy the refinery's other half, catapulting the deal's final price to $1.25 billion.

Petrobras Chief Executive Maria das Graças Foster, recently told a senate committee that the deal is being written off the company's books, a sign that "it wasn't a good deal" in retrospect.

The controversy surrounding Petrobras comes as Brazil is kicking off a presidential campaign. Ms. Rousseff is seen as the likely winner but her approval rate is declining. It was at 47.9% in the same CNT/MDA poll released Tuesday, down from 55% in February.

The senate investigation into Petrobras could drag on for months and into Ms. Rousseff's re-election campaign.

Her main opponents are already saying that the Pasadena deal shows that Ms. Rousseff is a weak manager, a politically sensitive claim since her managerial skills have been one of the PT's strongest selling points.

The CNT/MDA poll showed that 44.8% of respondents consider Ms. Rousseff to be a "regular" manager, while 30.6% said she isn't a good manager and 22.3% said she is a good manager.

Moreover, 80.5% of respondents who said they are following the Pasadena-related news said they believe there were "irregularities" in the deal, and 66.5% said that Ms. Rousseff is responsible and should have informed herself better before approving the acquisition.

The poll didn't connect voters' reaction to the Pasadena deal directly to Ms. Rousseff's approval rates or her chances of winning a second term this year.

Although Ms. Rousseff still appears as the likely winner in October, her lead has diminished. She is expected to get 37% of the vote, compared with 43.7% in February. Her top opponent, Senator Aécio Neves, from the social-democratic party PSDB, would get 21.6%, up from 17% in February. Mr. Neves has been leading the opposition's attempt to open an investigation commission for Petrobras.

The poll released Tuesday interviewed 2,002 people between April 20 and 25. It has a margin of error of 2.2 percentage points.

Brazil's stock market and Petrobras' shares began to rally after the poll results were out at 9:30 a.m. ET.

Analysts have said that any news indicating a potential change in the government could mean less intervention in state-controlled companies such as Petrobras, which has a big weight in the local stock market.

"We were surprised to hear a large number of investors and political analysts claim, with some conviction, that they believe that President Rousseff will lose her bid for re-election," Nomura's New-York-based economist Tony Volpon wrote in a note to clients released before the CNT/MDA poll. He said he was recently in Brazil and is "more sanguine" than the markets about Ms. Rousseff's re-election.

The Ibovespa stock index was up 0.76% minutes ahead of the poll's release and was up 1.55% half an hour later. It closed up 0.89%, at 51838 points. Petrobras was trading at 16.60 Brazilian reais ($7.45) a share before the poll, and at 17.07 reais after it, closing at 16.69 reais, up 0.79%.

Write to Paulo Trevisani at paulo.trevisani@wsj.com

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