Her centrist opponent, Jose Serra, is a former state governor and health minister who was routed by Lula in the 2002 presidental elections.
Rousseff is Lula's selected candidate. Correspondent Tom Phillips says her victory would largely be due to Lula's popularity.
"She's a very, very dour technocrat, she's not known to have the sort of personality like Lula," says Philips. "Lula's really hit the campaign trail over the last couple of weeks, which has really given her a boost. Most Brazilians have really not been particularly excited by either of the candidates this year."