(Updates with quotes from party official, context)
By Maria Carolina Marcello
BRASILIA, July 16 (Reuters) - The speaker of Brazil's lower house of Congress said on Thursday he is weighing legal arguments to impeach President Dilma Rousseff, which would be the first ouster of a sitting president in the country in nearly 23 years.
Eduardo Cunha, a leader of the PMDB party that is in charge of deciding whether to move ahead with impeachment, said such proceedings would be a step backwards for democracy, but added he expects a legal analysis on the matter within 30 days.
"I requested another legal analysis and I'm waiting to read that myself and come to a conclusion," Cunha told journalists.
Cunha had dismissed previous requests for impeachment out of hand, but calls are mounting from opposition lawmakers to unseat Rousseff for alleged campaign finance irregularities tied to a bribery scandal at state-run oil firm Petrobras.
Cunha has himself been named in a Supreme Court investigation of corruption at Petrobras, formally known as Petroleo Brasileiro SA.
Although his party is part of the governing coalition, Cunha has dealt Rousseff a series of embarrassing defeats in Congress and his opposition has grown more strident as the chances of formal charges has risen.
At a breakfast with journalists in the capital, Cunha even entertained the idea that his large, centrist PMDB party could exit the governing coalition before the next election in 2018, saying such a move was "not impossible."
In April, Cunha had rejected outright the possibility of impeachment proceedings against President Rousseff, who narrowly won re-election in October but has seen her popularity plunge due to the corruption scandal and an economic slowdown.
Federal prosecutors allege that a cartel of suppliers at Petrobras have overcharged the company billions of dollars for major contracts in return for kickbacks to executives and politicians, including some in Rousseff's Workers' Party
Federal police arrested the treasurer of the Workers' Party in April on charges that he solicited political donations stemming from bribes in the Petrobras scheme.
News magazine Veja reported last month that the executive accused of leading the alleged engineering cartel had confessed to making contributions to Rousseff's 2014 campaign with money from overpriced contracts. Rousseff and her party have denied any wrongdoing.
On Tuesday, police carried out their first search and seizure operations aimed at sitting lawmakers in the Petrobras probe, seizing luxury cars from the home of Senator Fernando Collor de Mello, a former president who was impeached in 1992 amid a corruption scandal. (Additional reporting by Alonso Soto and Silvio Cascione; Writing by Brad Haynes; Editing by Marguerita Choy and Alan Crosby)