Find the other articles in this series here: Part 1, Part 2, Part 3, Part 5, and a brief update. And the final update—Why Joni Ernst will be the next US Senator from Iowa.

For Part 4, I want to take a slight detour to examine whether presidential approval ratings could potentially influence the Iowa Senate race. Tomorrow, we’ll have a final look at the turnout model developed yesterday to determine under what conditions we can expect either a Democratic or Republican victory.

It is very early to make any assumptions about President Obama’s approval ratings six months from now, though he has been under water for the past year. It’s possible there could be a rebound before the election, or he could sink slightly lower. While there is often an assumption that presidential approval ratings have an effect on turnout down-ballot, it is necessary to test this hypothesis to see if it is true in Iowa.

It was necessary to test this in a different manner than the other analyses presented in this report. The White House has changed parties twice during the time period covered in the data. In order to get a sample size large enough to draw any conclusions, registration and turnout, along with Democratic and Republican approval, were combined into single variables. This allowed for an examination of same-party versus opposition-party effects on turnout.

As expected, presidential approval had a statistically significant, positive impact on same-party turnout. A higher approval rating will increase turnout for the president’s party. Surprisingly, there was no effect for the opposition party. In this case, a change in presidential approval did not impact turnout. It should also be noted that while it was challenging to include presidential approval in the turnout model for the reasons discussed, tests were run on partisan Congressional approval. These did not prove statistically significant in the turnout model, though it is correlated to registration, which is in the model.

While Republicans are gaining confidence heading into the midterms as Obama’s approval ratings sink, these results leave the effect on the race unclear. Democratic turnout will likely be depressed if the President’s approval rating remains low, but Republicans will be unlikely to make gains as a direct result of the President’s unpopularity. They still need to convince voters they offer a better alternative.

Figure 1