By Anthony Boadle
BRASILIA (Reuters) - Support for President Dilma Rousseff among Brazilian voters has fallen and her main rivals have gained ground, reducing her chances of an outright win in the October 5 election, a new poll by Datafolha showed on Friday.
The poll showed support for Rousseff slipped one percentage point to 37 percent, while Aecio Neves added four points to 20 percent and Eduardo Campos increased one point to 11 percent compared with its April poll, Datafolha said.
Rousseff´s comfortable lead at the start of the year has been undermined by concerns over rising inflation in a slow economy and political scandal surrounding management of Brazil´s largest company, state-run Petroleo Brasileiro SA.
Brazil's Congress is expected to open an investigation next week into the controversial purchase of a refinery in Texas and allegations of bribery involving SBM Offshore NV, a Netherlands-based supplier of offshore oil vessels.
Rousseff chaired Petrobras' board at the time and the allegations have hurt her reputation as a good manager. The congressional inquiry is expected to complicate her campaign.
If the vote were held today, Datafolha said, Rousseff´s opponents and other candidates would win enough votes to force a run-off, with a combined voter intention of 38 percent. This outcome is within the poll's margin of error of plus or minus two percentage points.
The left-leaning Rousseff would still win re-election in a second round vote by gaining 47 percent of the votes against 36 percent for Neves, the leader of the main opposition party, the centrist PSDB, the poll showed.
Brazilian stocks, especially shares of state-run companies, have risen in the wake of falling support for Rousseff, as investors critical of heavy-handed government intervention in the private sector hope for a more business-friendly administration in the future.
The decline in support for the president has led factions within the ruling Workers' Party (PT) to call for former president Luiz Inacio Lula da Silva, Rousseff's predecessor and mentor, to run in her place to ensure the party stays in power.
Datafolha said 58 percent of those polled thought Lula should be the PT's candidate. Among PT supporters polled, 75 percent want to see Lula run again.
The poll of 2,844 people was conducted on May 7 and 8 and published by the Folha de S.Paulo newspaper.
(Reporting by Anthony Boadle Editing by W Simon and Chizu Nomiyama)