Rousseff, former chief of staff to President Luiz Inacio Lula da Silva, fell 4 percentage points to 51 percent of the valid votes, according to a survey by the Ibope polling firm released hours before voting begins on Sunday. Another survey by pollsters Datafolha showed her support down slightly to 50 percent from 52 percent, although that decline was within the margin of error. Rousseff needs more than 50 percent of votes to avoid a runoff on October 31 against Jose Serra, a former Sao Paulo governor, who was barely changed at 31 percent in both new polls. All polls show Rousseff easily winning a runoff against Serra, who has run a lackluster campaign that has failed to energize voters. Rousseff would be the first woman elected president of the Latin American giant, thanks largely to Lula's support and his high popularity amid an economic boom. But the career civil servant's strong lead in polls over Serra has been threatened by ethics allegations against a former aide and the ruling Workers' Party. Rousseff is also facing a late slide in support among Brazil's millions of evangelical Christians as pastors throughout the country have railed against her for past comments advocating the decriminalization of abortion. The defections appear to have bolstered Green Party candidate Marina Silva, an evangelical Christian who has risen in some recent polls. The new Ibope poll surveyed 3,010 people on Friday and Saturday and had a 2-point margin of error. Datafolha polled 20,960 voters on Friday and Saturday, also with a 2-point margin of error.