(Updates with market reaction, analyst comments)
By Anthony Boadle
BRASILIA, Aug 18 (Reuters) - Marina Silva's entry into Brazil's presidential race will almost certainly force the October election into a second-round runoff and the environmentalist could even unseat President Dilma Rousseff, according to a poll released on Monday.
It showed Silva with the support of 21 percent of voters, almost three times more than center-left candidate Eduardo Campos, who she is poised to replace on the Brazilian Socialist Party's ticket after his death last week in a plane crash.
Support for Rousseff in the survey by polling firm Datafolha was unchanged from last month at 36 percent and remained at 20 percent for centrist and market favorite Aecio Neves, showing that Silva's surge came among voters who were previously undecided.
The poll showed Silva depriving Rousseff of the votes she needs to win the election outright in the first round of voting on Oct. 5. It also showed Silva ahead of Rousseff by 4 percentage points if there is a runoff between the two.
If Silva can sustain that momentum, she could pose the biggest threat to the ruling Workers' Party since it was voted in under Luiz Inacio Lula da Silva almost 12 years ago.
Brazil's stocks and currency gained on Monday as investors welcomed the prospect that Rousseff could be defeated, ending interventionist policies that have soured business confidence in Brazil's once-booming economy.
The poll was not all bad news for Rousseff, however. It showed that her government's approval rating rose six percentage points to 38 percent, probably due to a slowdown in inflation, which is a major concern for voters.
Silva's numbers may have been given a short-term boost by public sympathy over Campos' death and analysts said it is still Rousseff's election to lose.
"You can't rule out a contender who is still in first place," said Thiago de Aragão, a partner at Arko Advice, a political risk consultancy in Brasilia.
"Rousseff has a bigger party structure, she is still leading, and she has more TV time and money. If she uses those advantages to the fullest, it's going to be hard to lose."
The poll raised red flags for Neves, who had been Rousseff's main challenger but is now one percentage point behind Silva.
"Neves is now fighting for survival and will have to step up his criticism of Silva for her environmental policies that worry investors," said Andre Cesar, a political analyst in Brasilia.
CHANGE OF STRATEGY
Rousseff will have to change her campaign strategy, which until now has focused on painting Neves as an elitist intent on undoing the social gains that poorer Brazilians have enjoyed in recent years, said Tony Volpon, head of emerging markets research at Nomura Securities.
This "us versus them" rhetoric, Volpon said in a note to clients, will not work with Silva, the daughter of illiterate rubber-tappers who used to be a member of the Workers' Party and also served as Lula's environment minister.
The Brazilian Socialist Party plans to formally launch Silva, who won 19.3 percent running for the Green Party in the 2010 presidential election, as its candidate on Wednesday. Silva had been Campos' running mate until he was killed in a plane crash on Aug. 13.
Tens of thousands of Brazilians gathered on Sunday in the northeastern city of Recife to bid farewell to Campos, a successful two-time governor of Pernambuco state.
The Datafolha poll 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 outline her policies.
Silva draws support from many disenchanted Brazilians who were not planning to vote but will do so now that she will be top of the ticket, Datafolha said.
Her rejection numbers are also much lower: 11 percent said they would never vote for Silva, versus 34 percent for Rousseff.
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 Democracy Party.
Silva's anti-establishment style has also endeared her to Brazilians who took to the streets last year to protest against corruption, the high cost of living and poor public services.
A devout Pentecostal Christian, she also has a loyal following among evangelical voters, an increasingly influential segment of the Brazilian electorate.
The poll of 2,843 eligible voters was conducted Aug. 14-16 in 176 cities and had a margin of error of 2 percentage points. (Additional reporting by Patricia Duarte; Editing by Todd Benson and Kieran Murray)