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By Brian Winter SAO PAULO (Reuters) - With Brazil's economy struggling, a scandal at its state-run oil company and nearly three-quarters of voters saying they want change from their government, President Dilma Rousseff looks vulnerable in her bid for re-election this October. Senator Aecio Neves and former governor Eduardo Campos, who are both running on centrist, pro-business platforms, have failed to make significant headway in polls and still badly trail the left-leaning Rousseff despite her recent struggles. A recent surge in Brazil's stock and currency markets - fueled by investors who are tired of Rousseff's heavy hand in the economy and are betting on a change in government - risks being overdone, or at least premature. The challengers' biggest problem, polls suggest, is an inability to win over Brazil's lower-middle class, which swelled during an economic boom last decade and now accounts for more than half of the country's 200 million people.