The post-Democratic convention bounce has not affected Presidential race dynamics. Where it may matter is at the Congressional level. The post-Labor-Day bounce has reached downticket races, a fact that has major implications for control of both the Senate and the House. Both chambers appeared to be on a knife edge in August, but for now that is no longer true. If the bounce sticks, the probability of retained Senate control by the Democrats is 88%, the same as the probability of the President’s re-election. But will it last?
On average, each party wins more of the vote at the Congressional level when it wins the presidency than when it loses. To what extent is it driven by explicit messaging by the parties during a campaign?
In August, I said that the addition of Paul Ryan to the Republican Presidential ticket was a game-changer – but not for Romney, who on statistical grounds was unlikely to catch up with President Obama. Ryan is chair of the House Budget Committee, and his nomination allowed the national conversation to center on GOP Congressional budget priorities. Representative Todd Akin’s remark about “legitimate” rape opened the way for social issues to enter the national spotlight.
The Republican convention was light on specifics and yielded no bounce or even a negative bounce. Budget and social issues were embraced by the Democrats at their convention. The result has been a Presidential bounce of about 20 electoral votes, equivalent to 1.5 percentage points in Popular Vote Meta-Margin (the shift in opinion necessary to create an electoral near-tie). This is not particularly notable, since it keeps the race within the 290-330 EV range that we have seen since July.
However, a new wave of polls is notable. It shows that for the Senate, a larger shift has occurred.
First, a caveat. In well-polled races, such as MA-Sen Warren (D) v. Brown (R-inc.), the bounce was sharp and directly associated with the convention, where Warren was a prominent speaker. In other cases it is not possible to say for certain when the shift occurred. Indeed, we still don’t have any post-convention Senate polls for North Dakota or Connecticut, and only one in Montana. But a shift is visible for multiple other candidates, suggesting that some national factor has been the driving force. The most plausible candidate is the Democratic convention.
The Senate picture looked like this just one week ago:
Projected from today’s standings*, the median predicted November result is now 52D/I to 48R, with a 95% confidence band of 49-54 D/I seats. The probability of retained Democratic control is 88%.
However, there is also the possibility that the current bounce is transient. In this case, polls will drift back toward the Republicans, in which case the probability could be as low as 60% based on last week’s data.
The shift has a weak tendency to be larger in more strongly Democratic states. Democratic candidates have benefited in WI (D-Baldwin vs. R-Thompson, +11% swing), MA (D-Warren vs. R-Brown, +8% swing), and IN (D-Donnelly vs. R-Mourdock, a +4% swing). States showing no real movement are VA (Kaine vs. Allen, +0.5% swing). and MO (D-McCaskill vs. R-Akin, -1% swing, i.e. toward Akin). This is consistent with a recent Pew Center finding of increased Democratic enthusiasm post-convention.
Now that we have evidence of downticket effects of the national race, what should the Presidential candidates do to help their party? In the Senate, the answer is to travel to places where they are running ahead of their side’s candidate. Team Obama should probably have the President (and Michelle Obama, Joe Biden, and Bill Clinton) spend time in MA, VA, and CT. Team Romney should send Romney and Ryan to IN, ND, and WI.
The Senate is not the only chamber whose polls have moved. House polls have gone in the same direction. More on that soon.
*Technical note: I calculate the histogram above under the assumption that all polling margins will move together between now and Election Day by an amount diff. Then I let diff vary from -3% to +3%, and sum over all the possibilities. This does not account for state-specific events (Akin’s comment being a prime example), but it is not that far from reality. (Fellow geeks, the MATLAB code is here if you want to play around. To allow for biased drift, set the variable bias to specify drift in one direction; for example, bias=-2 indicates average movement of all margins toward Republicans by 2%.)