(Corrects date in dateline to Aug 18)
BRASILIA Aug 18 (Reuters) - Environmentalist Marina Silva will enter Brazil's presidential race in a strong second place, backed by 21 percent of voters, and could overtake President Dilma Rousseff in a second-round runoff, a poll showed on Monday.
Silva has drawn almost three times more support than the late center-left candidate Eduardo Campos who she is poised to replace in the race after his death last week in a plane crash.
The contest is being closely watched by investors weary of slow growth and state interference in Latin America's largest economy under the leftist Rousseff, and they are hoping to see a more market-friendly leader take office.
Support for Rousseff was unchanged at 36 percent, according to the Datafolha poll.
It showed that Silva would deprive Rousseff of the votes needed to win the Oct. 5 election outright and surpass her in a second-round runoff by 47 percent against 43 percent, within the poll's margin of error.
Silva, who won 19.3 percent in the 2010 presidential election as the Green Party candidate, is now outstripping centrist and market favourite Aecio Neves, who remained at 20 percent.
Campos was killed in an Aug. 13 plane crash and his Brazilian Socialist Party plans to launch Silva as its presidential candidate on Wednesday.
Tens of thousands of Brazilians gathered on Sunday in the northeastern city of Recife to bid farewell to Campos, a popular state governor whose death could initially give Silva's campaign a boost.
The poll conducted Aug. 14-16 was the first based on Silva's expected election run and could be skewed by a sympathy vote. The numbers could change when Silva hits the campaign trail and begins to define her policies.
Silva draws the support of many disenchanted Brazilians who were not planning to vote but will do so now that she will be top of the ticket, Datafolha said. If she does not run, Rousseff would win re-election in the first round, it said.
Silva appeals to young voters disillusioned with Brazil's establishment and seeking an alternative to the two-decades-old rivalry between Rousseff's Workers' Party and Neves' Brazilian Social Democratic Party.
The daughter of illiterate rubber-tappers who turned to politics to defend the Amazon rainforest, Silva has an anti-establishment style endearing to Brazilians who took to the streets last year to protest against corruption, the high cost of living and inadequate public services.
A devout Pentecostal Christian, she also has a loyal following among evangelical voters, an increasingly influential segment of the Brazilian electorate.
Silva's conservationist views have put her at odds with Brazil's powerful agribusiness lobby, though her economic advisers say she has become more pragmatic since she held up licenses for roads and hydroelectric dams in the Amazon region as environment minister in the first Workers' Party government.
While Silva is an unknown quantity to many investors, they would rather see her win than Rousseff.
The prospects of a Rousseff defeat drove up the Sao Paulo stock market last week, especially share prices for state-run companies that have lost market value during her administration marked by state intervention in the economy.
The poll of 2,843 eligible voters had a margin of error of 2 percentage points and was published on the website of the Folha de S.Paulo newspaper. (Reporting by Anthony Boadle; Editing by Mark Heinrich)