A big reason we pay so much attention to a president’s job approval rating, even in a midterm election when the president isn’t on the ballot, is the significant correlation between voter opinions of a president and their vote choice in an election. Suppose you think the president is doing a good job. In that case, you are almost certain to vote for a candidate from his party. In contrast, if you dislike the job the president is doing, you are equally certain to vote for a candidate from the other party.