(Corrects date in dateline to Aug 18)
BRASILIA, Aug 18 (Reuters) - Environmentalist Marina Silva will enter Brazil's presidential race in second place with the support of 21 percent of voters and would be tied with 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-leftist candidate Eduardo Campos, who she is expected to replace in the race after his death last week in a plane crash.
Support for the leftist Rousseff was unchanged at 36 percent, according to the 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 of the votes against 43 percent, within the poll's margin of error.
Silva, who won 19.3 percent in the 2010 presidential election, is now polling ahead of centrist Aecio Neves, the market favorite, 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.
The poll conducted Aug. 14-16 was the first based on Silva's expected entry into the race and could be skewed by a sympathy vote. The numbers could change when Silva hits the campaign trail and begins to define her policies. (Reporting by Anthony Boadle; Editing by Mark Heinrich)