Answer:
Midterm elections occur between two presidential elections, to elect a third of the Senate and a new House of Representatives. The midterm elections are often a referendum on the President, so if the President's party controlled all of the power in both houses of Congress, and the public is starting to disapprove of how the President is doing, the citizens can elect a new Congress that is in the opposition party. This will force the President to try to work with the opposition, and possibly make compromises on policy to get stuff done. (this is the right answer but it rarely happens in real life, they almost never compromise unless they absolutely have to)
