By Anthony Boadle BRASILIA (Reuters) – For the first time since a corruption scandal erupted at Brazil’s state-run oil company Petrobras last year, senior opposition politicians are publicly floating the possibility of impeaching President Dilma Rousseff. She does not face an immediate risk of impeachment, leading opponents told Reuters, but that could change if prosecutors find evidence that Rousseff knew of or benefited from the massive graft scheme. Prosecutors say executives at Petroleo Brasileiro SA , as Petrobras is formally known, conspired with contractors to skim billions of dollars for themselves and political parties when Rousseff was the company’s chairwoman from 2003 to 2010. The scheme continued after she became Brazil’s president in 2011.