(Updates with new poll data)
By Anthony Boadle
BRASILIA, Sept 23 (Reuters) - President Dilma Rousseff has regained a slight edge in Brazil's presidential race after weeks of aggressive campaigning against environmentalist Marina Silva, who is losing some voter support in the campaign's homestretch, polls showed on Tuesday.
Rousseff and Silva are tied at 41 percent in a likely second-round runoff that is expected to decide the election, according to a survey by the Ibope polling firm.
Another survey by MDA showed Rousseff would get 42 percent of the votes against 41 percent for Silva if the runoff were held today, a one point advantage that amounts to a statistical tie as it is within the poll's margin of error.
Silva, who would be Brazil's first black president, was leading polls on the runoff before the Rousseff campaign unleashed a wave of negative campaign ads questioning Silva's ability to lead Latin America's largest economy.
Anxiety about Rousseff's recovery in the polls has weighed on Brazilian financial markets in recent days. Sao Paulo's Bovespa stock index lost more than 1 percent on the poll numbers and Brazil's currency weakened past 2.40 per dollar for the first time in seven months.
After four years of sluggish growth and heavy-handed state intervention in the economy under the left-leaning Rousseff, investors are hoping the election will bring in a new president who will push for pro-market reforms that economists say are needed to lift Brazil out of its current rut.
Silva, a former senator and environment minister, surged in the polls after being thrust into the race last month following the death of her party's original candidate in a plane crash. She had been Vice President on the ticket before the crash. Recent polls have showed her support eroding, but she still looks like the best-placed challenger to unseat Rousseff.
"This week will be critical to see if Rousseff still has momentum to flip the lead," said Joao Augusto Castro Neves of the Eurasia political risk consultancy, which views Rousseff as the favorite to win.
In Tuesday's MDA poll, Silva's support in a first-round vote scheduled for Oct. 5 dropped six points to 27.4 percent, whereas Rousseff slipped just two points, to 36 percent.
The Ibope poll showed Rousseff picking up two percentage points to 36 percent in a first round and Silva slipping one percentage point to 29 percent. Support for Neves, a pro-business centrist who has been stuck in third place, was unchanged at 19 percent.
If no candidate wins an outright majority in the first round, the election will be decided in a runoff between the top two vote-getters on Oct. 26.
A longtime environmentalist who vows to transcend politics as usual, Silva's candidacy has generated excitement in a country where disgust over corruption, political horse-trading and poor public services led to mass protests last year.
She has also embraced market-friendly economic policies, winning her important allies in a business community that is frustrated with Rousseff's handling of the economy and 12 years of Workers' Party rule.
Rousseff has sought to use Silva's economic views against her, painting her as the candidate of a greedy financial elite with little concern for the poor. The Neves campaign has also focused attacks on Silva, hoping to woo back enough voters to make it into a runoff against Rousseff.
Tuesday's Ibope poll surveyed 3,010 respondents nationwide between Sept 18-23 and has a margin of error of plus or minus two percentage points. The MDA poll, which was commissioned by the transport industry lobby CNT, surveyed 2,002 people between Sept. 20-21 and has a margin of error of 2.2 percentage points. (Editing by Todd Benson, W Simon and Andrew Hay)