Brazil's tourism minister has resigned, in a move that adds pressure to embattled President Dilma Rousseff.
Opposition lawmakers want to remove Ms Rousseff over claims she manipulated accounts to hide growing deficit.
Officials from her coalition allies, the Brazilian Democratic Movement Party (PMDB), will vote to leave the alliance on Tuesday, members said.
Tourism Minister Henrique Eduardo Alves became the first PMDB member to stand down from government on Monday.
Ms Rousseff, a former political prisoner during Brazil's military government, began her second term in office 14 months ago.
But her popularity has plummeted amid corruption allegations around senior members of the governing Workers' Party.
The speaker of the lower house of Congress, Eduardo Cunha, agreed in December to open impeachment proceedings against her.
Last week, Ms Rousseff, who denies wrongdoing, said the procedure amounted to a coup. She spent Monday meeting officials from the PMDB ahead of that party's national leadership meeting on Tuesday.
But a number of MPs from the PMDB said ahead of the meeting that most members had already decided to abandon the coalition.
"On Tuesday we will be disembarking from this government," Senator Valdir Raupp told Reuters.
The PMDB is headed by Michel Temer, Ms Rousseff's deputy, who would become president should she be removed.
The loss of support by his party's 69 MPs could have consequences for the impeachment proceedings. Ms Rousseff needs the support of a third of the 513 members of the lower house of Congress to stave off impeachment.
The Workers' Party has been in power since Luiz Inacio Lula da Silva was sworn in for his first term in 2003.
It has been hit by a long-running investigation into bribes from contractors working for state oil company Petrobras.
A recent attempt by Ms Rousseff to appoint Lula as her chief of staff was seen by critics as an attempt to shield him from money-laundering charges - which he denies - connected with the case.
His appointment was blocked by a judge earlier this month.
Lula said on Monday he expected his successor to survive growing pressure, and said he would speak to Mr Temer to work out how to save her job.
Protests involving tens of thousands of people have taken place across Brazil to call for Ms Rousseff's impeachment.
According to poll by the Datafolha poll in late February, only 11% of respondents across the country said the president's performance was "good or excellent".