Trump could cost GOP control of Congress, historical analysis shows

Midterm elections have generally come to be seen as the electorate’s reaction to a presidency. But this one is on a whole different level. “In no previous election,” Gary Jacobson, a University of California political scientist who crunched the numbers, tells me, “has the linkage between opinions of the president and how people are likely to vote been as strong as this time.” Jacobson’s research goes back to the 1930s, before which there was no polling and therefore no ability to compare.

Jacobson, who presented his findingsto the American Political Science Association recently and provided me with updated data, found in 93.1 percent of cases this year, voters’ approval or disapproval of the president is correlated with their planned votes for or against the president’s party in House races. That’s an all-time high. It averaged 86 percent in recent elections, 74 percent in the 1980s and 1990s.

And it’s more than a casual correlation. Using regression analysis, Jacobson determined that for every percentage point movement in Trump’s job approval rating, support for Republican House candidates in the midterm elections move by 0.75 percentage points — the highest effect ever seen. For Barack Obama, it was 0.50 percentage points. For George W. Bush and Bill Clinton, closer to 0.25 percentage points. There isn’t as much data about Senate voting, but the relationship has been virtually identical.