President Dilma Rousseff's chances of surviving impeachment are growing slimmer now that three parties have decamped from her governing coalition in 24 hours and Brazilian legislators begin preparing for a crucial vote on the question expected this weekend.

The pullouts of the mid-size centrist parties caused despondency among members of Rousseff' left-leaning Workers' Party ahead of the vote expected Sunday in the Chamber of Deputies. That vote will determine whether the impeachment process moves ahead based on allegations that Rousseff's administration violated fiscal rules.

The pro-impeachment camp needs two-thirds of the 513 votes in the lower house, or 342 votes, to send the proceedings to the Senate for a possible trial. According to a tally by the newspaper Folha de S. Paulo, 284 legislators have come out for impeachment, while 114 are opposed and 115 are undecided.

While the outcome is still too close to call, the pullouts by the majority of the 36 deputies for the Social Democratic Party, the Progressive Party with 47 deputies and the Brazilian Republican Party with 22 legislators made it that much harder for Rousseff to defeat the vote.

In a rare bit of positive news for the government, leaders of the Democratic Labor Party pledged to cast their party's 20 votes against impeachment.

But the loss of three more parties was a blow for the governing coalition, coming about two weeks after the Brazilian Democratic Movement Party, the country's largest party, also quit. It has 66 deputies in the lower house.

"Ministers close to Dilma believe the battle against impeachment is virtually lost," Monica Bergamo, a respected political columnist for Folha de S. Paulo, wrote Wednesday. "Not all of them have definitively thrown in the towel, but the consensus is that the government is going through its worst moment."

Rousseff has seen her approval ratings tumble amid the worst recession in decades, a spike in both joblessness and inflation, and a sprawling probe of corruption at the state-run energy company Petrobras, which over the past two years has ensnared dozens of top politicians across the political spectrum as well as some of Brazil's richest and most powerful business executives.

The president appeared simultaneously defiant and conciliatory Wednesday, sketching out her plans for "once this page has been turned." Speaking at an event about the modernization of Brazil's port infrastructure, she pledged that "starting next week," her administration would reach out to even its foes in a bid to get the country back on track.

Ricardo Berzoini, Brazil's political affairs minister and a Rousseff ally, said at a news conference that the government could file a request for the Supreme Court to intervene in the impeachment effort later Wednesday or Thursday.

"We can't play around with democracy," he said. Berzoini also said Rousseff is meeting with lawmakers one on one to make her case.

Chamber of Deputies officials announced the order in which votes are to be cast in Sunday's vote, which is in line with a plan by Speaker Eduardo Cunha, a Rousseff foe who has been the driving force behind impeachment. The vote will begin with representatives from Brazil's southern states, which tend to be strongly anti-Rousseff, and work up to the largely pro-government states at the end, in an apparent bid to create the impression of an unstoppable wave of support for impeachment.

The newspaper O Globo in Rio de Janeiro said that under that system, the first deputy to vote would be Afonso Hamm, a Progressive Party legislator from the southern Rio Grande do Sul state. Hamm is under investigation by Brazil's Supreme Court for alleged involvement in the sprawling corruption investigation at the state-run Petrobras oil company.

The impeachment proceedings against her stem from allegations her administration violated fiscal rules to mask budget problems by shifting around government accounts. Opposition parties claim sleight-of-hand accounting moves allowed her to boost public support and say impeachment is in line with the wishes of the majority of Brazilians.

Rousseff and her supporters say the allegations are bogus and insist financial maneuvers like the ones she made are common practice, used by two prior presidents. She has repeatedly denounced the proceedings as an attempted coup and a blatant power grab by her foes.

If the pro-impeachment camp wins Sunday's vote, the proceedings move to the Senate, where a vote would determine whether to begin a trial against Rousseff. If that happened, she would immediately be suspended from office and Vice President Michel Temer would take over. Temer has also been mentioned in plea bargain testimony in the Petrobras case.