Campos, 49, died Wednesday with six others when his plane, on its way to a campaign stop, crashed in a residential neighborhood of the city of Santos, near Sao Paulo. Several people on the ground were injured, according to Agencia Brazil, the state-run news agency.
He was polling in third place with about 10 percent of voters' preferences, behind Rousseff of the Workers' Party with 36 percent, and center-right Social Democracy Party candidate Aecio Neves with 20 percent. Neves is the grandson of Tancredo Neves, who won Brazil's 1985 presidential election, but because of illness was never sworn in as president. Eight other candidates are polling three percent or less.
By law, Campos' party has 10 days to name a replacement, and it is likely to be Marina Silva, Campos' vice presidential running mate, analysts say.
Silva, a former environment minister with a strong record of defending Brazil's environment, is seen as less pro-business than Campos and more likely to engage undecided voters. She is also popular among young, urban voters, and her potential strong showing in the Oct. 5 election would come at the expense of Rousseff.
A runoff election would be scheduled if no candidate receives more than half the votes, and polls suggest a tight race between Rousseff and Neves if the election goes to a second round.
"Now it's likely that Neves could see even more votes coming from supporters of Silva after the first round," said Thiago de Aragão of the Brazilian political consulting firm Arko Advice. "The campaign will now be harder for Dilma (Rousseff), increase chances for Neves, and add a heavy emotional characteristic in the voting process."
After the Brazil successfully hosted the World Cup, Rousseff saw increases in her popularity, but her approval ratings are lower than a year ago. Citizens have protested high inflation, poor public services and slow economic growth, and support from Brazil's business sector has been eroding as well.