Trende created the chart above on “the relationship between presidential job approval in the final Gallup poll before midterm Election Day and the share of the president’s party’s congressional delegation that went down to defeat”:
This isn’t a perfect relationship, but presidential job approval is still the most important variable for how his party fares in midterm elections, explaining about half of the variance. The relationship is highly statistically significant: For every point in job approval the president loses, his party loses 0.6 percent of its caucus. (The chart doesn’t measure drop in job approval; just job approval.) So, at 60 percent, the president should lose 5 percent of his caucus; at 50 percent, it is around 12 percent of his caucus lost; at 40 percent, it’s about 18 percent of his caucus lost — which would be 36 seats.
Now the latter is highly unlikely to happen. To pick up 36 seats, the GOP would have to win every seat that Obama won with 56 percent of the vote or less in 2012. Right now the GOP only holds five seats the president won with 54 percent of the vote or more, and only one seat he won with over 56 percent of the vote.