Ms. Rousseff won a second term less than a year ago, but she is now struggling with sharply circumscribed powers. Some of Brazil’s most powerful men are pushing her into a corner, defecting from the governing coalition and greatly reducing her ability to pass legislation, while her own vice president signals that his time to lead may be coming.

Ms. Rousseff’s adversaries include figures who have been tarnished by the Petrobras graft scandal, like Eduardo Cunha, the speaker of the lower house, who recently broke from her coalition, and Fernando Collor de Mello, the former president who resigned in 1992 under suspicion of corruption and later resurrected his career as a senator.

Image Dilma Rousseff Credit... Evaristo Sa/Agence France-Presse — Getty Images

Opposition legislators are seeking to start impeachment proceedings against Ms. Rousseff. The president seemed to have a respite this week when Renan Calheiros, the head of the Senate, who is also under investigation in the Petrobras scandal, expressed opposition to impeaching her. But in doing so, Mr. Calheiros managed to increase his own bargaining power, with Ms. Rousseff speaking favorably about his proposals to mend the economy.

Only months after her re-election, Ms. Rousseff has watched her approval ratings plunge into the single digits in 2015, the lowest of any Brazilian president in decades, as the economy comes under stress, with the gross domestic product seen shrinking more than 2 percent this year.

But a declining economy is not an impeachable offense, and no testimony has surfaced suggesting that she received any bribes, complicating the efforts to oust her. The courts are examining whether Ms. Rousseff’s campaign received illicit donations and whether she improperly used funds from state banks to cover budget shortfalls.

Either way, many voters hold her responsible for failing to curtail corruption in her own government and for policies viewed as worsening the economic slump. A successful but poisonous re-election campaign last year may be partly to blame, when Ms. Rousseff disputed the depth of economic problems and rejected her opponent’s proposals for fixing them.