Brazil's President Dilma Rousseff walks past Brazil's Chief of Staff Aloizio Mercadante during a ceremony to reappoint Rodrigo Janot to the position of Prosecutor-General of the Republic at the Planalto Palace in Brasilia, Brazil, September 17, 2015. REUTERS/Ueslei Marcelino BRASILIA (Reuters) - Brazil's opposition parties on Thursday filed a request in Congress to impeach President Dilma Rousseff for breaking fiscal rules by allegedly manipulating government finances to benefit her re-election last year.

The request was backed by some members of Rousseff's main political ally, the fractious PMDB party, Brazil's largest, whose votes would be needed to succeed in ousting the president.

The most serious move so far to seek her impeachment is based on the findings of a federal audits court that said the Rousseff administration doctored government accounts last year to allow more public spending in the run-up to the October election, in violation of Brazil's fiscal responsibility law.

The request accuses the president of signing decrees to authorize public spending behind the back of Congress in an election year.

Just six months into her second term after her narrow re-election, Rousseff is facing mounting calls for her resignation or impeachment. She has been weakened by a massive bribery scandal that has implicated members of her coalition, and by Brazil's slide into the worst recession in decades.

Opinion polls show two in three Brazilians want to see her impeached but it is not clear that the opposition has the political clout to trigger impeachment proceedings in the lower house of Congress.

Rousseff's opponents say they have 280 votes but would need two thirds of the house, or 342 votes, to open an impeachment trial that would be held in the Senate.

"The opposition on its own cannot get impeachment approved in the lower chamber," said Senator Ronaldo Caiado of the center-right Democrats, an opposition party.

Impeachment proceedings in the lower house depend on Speaker Eduardo Cunha, a fierce enemy of the president who broke from her coalition this year to join opposition efforts to obstruct her legislative agenda.

A man carries a poster showing support for Brazil's President Dilma Rousseff during the March of the Daisies, calling for improved rights for women working in rural areas and forests, in Brasilia August 12, 2015. Reuters

In a sign of her deteriorating situation, the request to Congress was made by Helio Bicudo, a prominent human rights activist during Brazil's military dictatorship in the 1970s and a founding member of her Workers' Party who left in 2005.

The request was drafted by another prominent lawyer, Miguel Reale, a consultant for the main opposition party, the PSDB.

"We fought the dictatorship of guns. Now we are fighting the dictatorship of bribes," Reale said at a news conference.

(Reporting by Anthony Boadle; Editing by James Dalgleish)