(Recasts first paragraph, adds latest poll data from Vox Populi and Ibope)
By Anthony Boadle
BRASILIA, Sept 23 (Reuters) - President Dilma Rousseff has regained the edge in Brazil's presidential race after weeks of aggressive campaigning against environmentalist Marina Silva, who has lost ground among voters in the campaign's homestretch, polls showed on Tuesday.
A survey by the closely watched Ibope polling firm showed Rousseff and Silva are tied at 41 percent in a likely second-round runoff that is expected to decide the election.
According to another smaller poll by Vox Populi, Rousseff has overtaken Silva and opened a seven-point lead in an expected second-round of voting.
"This week will be critical to see if Rousseff still has momentum to flip the lead in more than one poll," said Joao Augusto Castro Neves of the Eurasia political risk consultancy, which views Rousseff as the favorite to win.
If no candidate wins an outright majority in the first round on Oct. 5, the election will be decided in a runoff between the top two vote-getters on Oct. 26.
Silva, who would be Brazil's first black president, was leading runoff polls 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 has lost more than 4 percent in the last five days on the poll numbers, and Brazil's currency weakened past 2.40 per dollar on Tuesday 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, many 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 the candidate for Vice President on the ticket before the accident. Recent polls have showed her support eroding, but she still appears to be the best-placed challenger to unseat Rousseff.
Silva's first-round support dropped six points to 27.4 percent, according to another poll on Tuesday from research group MDA, whereas Rousseff had between 36 and 40 percent of the vote in Tuesday's polls.
Aecio Neves, a pro-business centrist who has been stuck in third place, was unchanged at 19 percent, according to Ibope, which was his best showing in the three polls released on Tuesday
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 and Vox Populi polls surveyed 2,000 people on Saturday and Sunday and both have a margin of error of 2.2 percentage points. (Additional reporting by Jeb Blount in Rio de Janeiro; Editing by Todd Benson, W Simon, Andrew Hay and Ken Wills)