A political crisis gripping Brazil will deepen if a major ally quits President Dilma Rousseff's governing coalition, as expected, a top Brazilian judge said Tuesday.

Brazil will become harder to run if the Brazilian Democratic Movement Party confirms later Tuesday it is withdrawing its support for Rousseff's troubled administration, Supreme Court Justice Gilmar Mendes said.

"The government's support base will become even narrower, and the political crisis will obviously worsen" if the ally walks out, Mendes told reporters during a visit to Lisbon, Portugal.

Mendes recently blocked former Brazilian President Luiz Inacio Lula da Silva's nomination to a Cabinet post due to ongoing corruption investigations, though that decision is under appeal. Rousseff had sought to shore up her government by bringing in Lula.

Mendes is attending a three-day legal conference in Lisbon, which began Tuesday, and includes participants seen as opponents of Rousseff. Senator Aecio Neves, who narrowly lost to Rousseff in the 2014 presidential runoff, is scheduled to speak Thursday.

About 50 people, mostly Brazilians, protested outside the event at Lisbon University's Law Faculty against moves to impeach Rousseff amid a huge kickback scandal involving her Workers Party. They called the impeachment an attempted coup and jeered Brazilian Senator Jose Serra, one of Rousseff's opponents, as he entered the building.