It’s been kicked around in punditland that party conventions have little value in modern political life. This is contradicted by the lasting change in President Obama’s lead over Governor Romney, as well as the notable increase in his party’s likelihood of retaking the House of Representatives. Now, reader Rick in Miami has done a nice bit of analysis that completes the picture. He finds that top-of-the-ticket moves are closely accompanied by parallel downticket effects in the Senate. Because of the convention and its aftermath in the Presidential race, Democrats are no longer at meaningful risk of losing the Senate.

Last week I posted my code for making the Senate prediction. Rick downloaded it and souped it up. Here’s what he found, in his own words:

I took your file senate.m and upped the “in play” seats to 14, and modified it to plot a time series (attached modification “senate2.m”).

The time series (in attached figure) shows that control of the Senate has been much more volatile than the presidential race, with the % chance of Dem control dropping sharply from about 80% in mid-August to below 50% in late August. That drop ended with Akin’s infamous comment. The meteoric increase in the second week of September – the week after the Dem convention – is extremely dramatic! I remember you posting that the Senate situation was changing, but didn’t appreciate the magnitude of that change until I plotted this time series.

Take a look:

The Democratic control probability is 94%. The median expected split is 53D/I, 47 R (68% confidence interval, 52-54 D/I). Now that the Senate has essentially left knife-edge territory, the one remaining knife-edge issue is the House of Representatives (D – ActBlue) (R – Crossroads GPS).

The remarkable similarity between this plot and the Presidential Meta-Analysis is most easily explained by partisan enthusiasm. Certainly the addition of Paul Ryan as VP was electrifying to the Republican base. In some sense, the Republicans got their convention bounce – two weeks before the convention. (I note that something similar happened in 2004 for the Democrats with John Edwards). Then, with the Akin remark and the Democratic convention, the continuing parallel path of President Obama and his party’s Senate fortunes confirms the idea that some shared cause is at work.

Before, I had been entertaining the idea that the conventions’ main effect was to change the minds of decided voters, à la the the RAND survey. But in the last few weeks, their survey shows a net total shift of only about 1.5% of margin toward Obama. Over the same period, the Meta-Margin grew by 3.0% and is still on the move.

Now we have a quantitative explanation for the remaining change. As I have written, Senate candidates received bigger bounces in strongly Democratic states. Combined with Rick’s analysis, I suggest that at least half of the bounce is caused by increases in partisan enthusiasm – an effect with repercussions up and down the ticket.

This analysis quantifies what the Democrats got out of their convention. For them, it was money and time well spent. As Rick wrote to me, it

puts a rather strong counterpoint to the idea that “the Conventions are outdated and unnecessary.”

Rick’s code is here (some assembly required).

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As a consequence of the change in Senate odds, I am unofficially retiring all Senate races as an ActBlue priority. The most effective place to donate is now towards House races, i.e. the DCCC. Senate races will still be shown with a ranking of relative importance, just de-emphasized. For Republicans, I suggest that you trust Uncle Karl.