By Brad HaynesSAO PAULO (Reuters) - Tens of thousands of demonstrators gathered across Brazil on Sunday to call for the impeachment of President Dilma Rousseff, whom they blame for a vast corruption scandal and the economy's worst slump in a quarter century.Less than a year into her tumultuous second term, the left-wing president's support has dwindled to single digits in recent polls, while two in three support calls for her impeachment.Sunday's protests were the third major demonstration of the year against Rousseff. Protesters swarmed to midday marches on Rio de Janeiro's Copacabana Beach and in front of Congress in the capital Brasilia, but a more tepid buildup on social media suggested the nationwide movement may not attract the millions of marchers touted by independent organizers in past days.Support for the protest movement remains widespread as rising unemployment and inflation presage the worst economic downturn since at least 1990. Government austerity efforts meant to keep the country's investment-grade credit rating have also turned off Rousseff's supporters and met resistance from lawmakers."We hear all about budget cuts, but so far it's only hurting people like me," said Francisco Mosack, a retired metalworker from the humble Sao Paulo neighborhood of Capao Redondo, who complained that his electric bill had doubled in a year."I'm coming out to show my indignation," said Mosack, 65, wearing the canary yellow jersey of Brazil's national soccer team as he joined a sea of green and yellow along Sao Paulo's midtown thoroughfare Avenida Paulista.POOREST TURN ON PRESIDENTA Datafolha survey this month showed impeachment has the most support among the poorest and least educated Brazilians, who overwhelmingly backed Rousseff in her narrow re-election victory last October.Sensing weakness, congressional leaders have sabotaged much of the president's legislative agenda. The leader of the lower house of Congress, who is responsible for moving ahead with an impeachment vote, broke entirely with her government last month.The president had measured success this week consolidating support among restive Senate leaders for a pro-business agenda. But their alliance remains fragile.Further muddying the waters, a bribery and money-laundering investigation at state-run oil company Petrobras has spilled into the political realm as the prosecutor-general readies charges against sitting politicians.Brazil's biggest-ever corruption scandal already landed several of the country's most prominent executives in jail, and plea deals have implicated congressional leaders and senior members of Rousseff's government.No evidence has emerged incriminating Rousseff, but many protesters consider her responsible for wrongdoing at Petrobras, where she was chairwoman from 2003 to 2010.(Additional reporting by Anthony Boadle and Luciana Otoni in Brasilia, Roberto Samora and Eduardo Simes in Sao Paulo; Editing by Mary Milliken)Join the conversation about this story Click here to read full news..