- Some call it the midterm curse.

In midterm House and Senate elections, the elections between presidential elections, the president's party usually gets thrashed.

Why does that happen?

Let's look through a quick history of midterms to get some answers.

(upbeat music) What's at stake?

1/3 of the United States Senate seats and all 435 House seats are on the ballot.

Since World War II, the average loss for the president's party is 30 seats in both houses of Congress.

With only a few exceptions, it's ugly.

So why do presidents suffer so?

First, there is the theory called the presidential penalty.

Voters who are angry or unhappy with the president tend to vote more than those who are satisfied.

The president's party loses.

Blame it on a bad report card.

Then there's the coattails effect.

When a popular president is elected, often other candidates in the same party are swept into office.

By the midterms, the coattails disappear, and many of those candidates lose.

The surge and decline theory is all about voter turnout.

There are simply more voters in presidential election years, thus the surge.

Then there is balance theory.

Some midterm voters simply want to be sure there is a balance of power between the parties in the White House and the Congress.

Sure, there are a few outliers of note.

200 years ago, Thomas Jefferson made gains at the beginning of an era of good feelings.

With the New Deal in 1934, Franklin Delano Roosevelt picked up 18 seats.

After 9/11, patriotic sentiment helped George W. Bush gain midterm seats.

There is no perfect predictor.

Either a president's high approval rating or a thriving economy can reliably foresee if there will be a positive midterm.

Incumbent Presidents may not like midterm curse, but for political scientists, it is a statistical dream, something you can count on happening nearly every time.

(upbeat music continues)