President Dilma Rousseff’s chances of surviving impeachment appeared diminished, with three parties decamping from her governing coalition in 24 hours as Brazilian legislators prepare for a crucial vote on the question expected this weekend.

The pullouts of the midsize centrist parties sparked growing despondency among members of Rousseff’s 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.