Rousseff's majority, already reduced by Green Party gains in the first round, appeared under increasing threat as opposition candidate Jose Serra rallied devout Roman Catholic voters skeptical of the ruling Workers Party candidate's credentials on abortion.
The abortion debate has already caused a split in the National Conference of Brazilian Bishops in the world's largest Catholic nation -- 85 percent of Brazil's 189 million people.
In an attempt at damage control, the national organization condemned bishops from its own South Region 1 for urging voters not to vote for Rousseff, protege of incumbent President Luiz Inacio Lula da Silva and a former Marxist guerrilla.
In a television debate last week Serra, former mayor of Sao Paolo, accused Rousseff of fudging on abortion after supporting it through much of her campaign in a bid to claw back her following among anti-abortion rights voters.
Rousseff's position was further challenged when the southern bishops joined in the row.
The latest opinion polls indicated gains by Serra that raised the prospect he could be closing the 14-percentage point gap of the first round before the runoff Oct. 31.
Gains were also reported by Green Party candidate Christian Marina Silva, an evangelist who is emerging as the key to the final outcome.
Although Rousseff argues the abortion issue is economic, not religious, the debate has shifted attention from the economy and other issues of national importance.
In a public reproof to the southern bishops, the justice and peace commission of the National Conference of Brazilian Bishops condemned the use of the Christian faith in the electoral process, expressing concern for the actions of many groups that, in the name of Christianity, "have created difficulties for a free and informed vote."
The national leadership condemned the text published by the region and stressed that it doesn't represent the mind of the entire Bishops' Conference. "The CNBB does not suggest any candidate and recalls that the choice is a free and conscious act of each citizen," it said.
The note added, "Faced with this great responsibility, we urge Catholics to consider ethical criteria, especially unconditional respect for life, family, religious freedom and human dignity."
Serra and Rousseff clashed over abortion in their first debate Oct. 10, when Serra quoted Rousseff's comment that abortion must be treated as a "public health issue" rather than a religious one. Rousseff accused him of slander.
Brazilian law allows pregnancy to be terminated only if it results from rape or it puts the mother's life at risk. But each year millions of Brazilians are known to have abortions.
The opposition and the religious lobbies have joined forces against Rousseff over her comments. "The question is whether we are going to be hypocritical and pretend not to see the 3 1/2 million women who perform abortions in precarious conditions," she said. "Are we going to arrest them or take care of them?"
Analysts said Brazilians' inability to grasp with the issue compared with the nation's attitudes toward chronic poverty. Tourists visiting Brazil are regularly given guided tours of favelas, shanty towns on the outskirts of high-rise urban centers. Rio de Janeiro is home to some of the densest favelas, hotbeds of crime and narcotic trade, some of which have become tourist destinations.