If anything, Brazilian President Dilma Rousseff is a survivor.

The former political prisoner has lived through cancer, endured raucous, anti-government protests and brushed past critics to pull off a successful World Cup. She's even held onto wide support as Brazil's economy sputtered into recession.

Sunday's presidential election determines the outcome of what may be her most surprising challenge yet: the unexpected rise of Marina Silva. The Amazon-born environmentalist was thrust into the presidential race when a plane crash killed her party's top candidate.

Just weeks ago, polls suggested Rousseff and Silva would advance to a runoff vote that Silva was likely to win. But Rousseff fought back aggressively by raising doubts about Silva's qualifications and the latest polls show Rousseff once again with a strong lead.