What usually happens to total seats in the house of representatives at the midterm elections?

The president's party usually loses seats in the house of representatives at the midterm elections. There are two main reasons why this occurs. The first reason is that turnout is usually lower for midterm elections and the composition of the electorate is generally different than during presidential years. During midterms the voter base is generally more partisan. The second reason is that US presidents almost always fall in the polls after they are elected which effects congressional candidates.