Brazilian President Michel Temer accepted the resignation on Friday of his minister of political affairs, whose party is abandoning the ruling coalition to ready for the 2018 election.
Antonio Imbassahy of the Brazilian Social Democratic Party (PSDB) has handled the government's relations with Congress, keeping an unwieldy coalition united in blocking the prosecution of Temer on corruption charges earlier this year.
However the PSDB, Temer's largest ally, has split over whether to stick with the unpopular president in the run-up to a general election next October, when the party plans to field its own presidential candidate.
The PSDB will hold its national convention in Brasilia on Saturday, when it is expected to elect Sao Paulo Governor Geraldo Alckmin as its new party leader, replacing Senator Aecio Neves, who is under investigation for graft.
That would make Alckmin the PSDB's most likely candidate in next year's presidential race.
Allies say Alckmin plans to continue his party's disengagement from the Temer government, where the PSDB still has two ministers, including Foreign Minister Aloysio Nunes, who is expected to stay on the job based on personal conviction.
Still, Alckmin is expected to mobilize PSDB support for Temer's signature legislation proposal, the overhaul of Brazil's costly social security system. Investors consider the measure crucial to closing a huge budget deficit that cost Latin America's largest economy its investment-grade credit rating.
Temer needs the PSDB's 46 votes in the lower house of Congress to win passage of the pension reform bill, which is set for a vote the week of Dec. 18.