RIO DE JANEIRO — The effort to oust President Dilma Rousseff of Brazil was thrown into chaos on Monday when the new speaker of the lower house of Congress annulled a vote to impeach her, immediately upending the power struggle gripping Latin America’s largest country.

The surprise move came just two days before the Senate is expected to decide on whether to suspend Ms. Rousseff, replace her with the nation’s vice president and put her on trial.

Senate leaders vowed to defy the decision on Monday, promising to decide the president’s fate this week anyway. Lawmakers on both sides of the issue said they would rush to the Supreme Court, hoping for an answer on whether the impeachment proceedings would move forward as planned.

And Brazilians were left, yet again, trying to make sense of the maneuvering by their scandal-plagued leaders.