* Gap between runoff rivals down to 2 points- Datafolha poll
* Poll suggests President unhurt by Petrobras bribes scandal (Adds bribery scandal, background)
By Anthony Boadle
BRASILIA, Sept 19 (Reuters) - Brazilian President Dilma Rousseff has clawed back support at the expense of her main opponent in next month's presidential election, a poll showed on Friday, suggesting the runoff that should decide the vote is too close to call.
Marina Silva surged in the polls after joining the race on the death of her party's original candidate last month, and she appeared poised to unseat Rousseff with a 10 percentage-point margin in the likely second round vote.
But Rousseff has since recovered ground, aided by a TV campaign that has questioned Silva's ability to govern Latin America's largest nation and portrayed her pro-market policies as favoring the rich and hurting the poor.
Friday's Datafolha survey showed that, in a simulation of the likely runoff, the gap between them had narrowed to two points - 46 percent for Silva and 44 percent for Rousseff. That is equivalent to the poll's margin of error.
Rousseff widened her lead over Silva in the first-round vote to seven points, raising her support to 37 percent from 36 percent in the previous poll, while Silva fell to 30 percent from 33 percent. Centrist candidate Aecio Neves, the market favorite, has risen to 17 percent from 15 percent.
If no candidate wins a majority in the Oct. 5 election, the race will be decided in a runoff three weeks later between the two top vote-getters.
Rousseff has promised to expand social programs that are widely acknowledged to have reduced poverty and inequality during 12 years of Workers' Party rule. But the left-leaning economist's policies of intervention in the economy have also drawn harsh criticism from investors.
Silva, an environmental activist turned anti-corruption campaigner, wants to break with Brazil's murky coalition politics to restore trust in government, a popular stance in a country where many voters are fed up with traditional parties.
The poll numbers suggest that Rousseff has not been hurt by a new bribery scandal involving state-run oil company Petroleo Brasileiro SA, which could derail her re-election bid if it snowballs.
A jailed former Petrobras executive, Paulo Roberto Costa, has named two dozen Rousseff allies, including her energy minister, for allegedly receiving kickbacks on contracts, according to a plea bargain statement leaked to local media.
In a new leak on Thursday, TV Globo reported that Costa told federal prosecutors that bribes were paid in the purchase by Petrobras of a refinery in Pasadena, Texas, and that he himself received 1.5 million reais ($634,000).
Dubbed the "ticking time-bomb" because of what he might reveal, Costa remained silent when summoned on Wednesday by a Congressional inquiry into the refinery deal.
The new poll surveyed 5,362 voters on Sept. 17-18. The poll was commissioned by TV Globo and the Folha de S.Paulo newspaper, which posted the results on its website. (Reporting by Anthony Boadle; Editing by John Stonestreet)