Measuring the effect of a particular political event is challenging. Any single poll spans multiple days, and multiple polls are necessary to get good accuracy. National surveys give the first indication, within a week. State polls (upon which the PEC snapshot and forecast are based) are more accurate when aggregated, but take longer.
In the case of the Republican convention, we will have a hard time knowing what its effects are in isolation. Certainly the event was distinctive. Last night, Donald Trump entered the general election campaign with a harsh 76-minute speech that painted the U.S. as a dystopia, and his opponent as a criminal. These claims do not hold up to scrutiny – but they do show his approach for the months ahead. One might like to know the net effect of that speech, that of his endorsers and various other Trumps, and of Ted Cruz, who called for citizens to “vote your conscience.” It would be interesting to know if such a convention would close his deficit with Republican voters.
Almost immediately, the Democrats now take the stage. Today, Hillary Clinton is expected to announce her vice-presidential pick. Next week comes her party’s convention. Many polling measurements will capture the combined effect of all of the Republican and Democratic events. The conventions are close in time, making it easy for viewers to see contrasts between the parties. My guess is that the net effect should be relatively large, favoring the Democrats. If I am wrong, undecideds should still decrease.
However, how large is “relative”? The effects of conventions have been declining. Based on Gallup data, the median “net impact” (more-likely-to-support minus less-likely-to-support) of conventions from 2004-2012 was 5 percentage points, compared with 16 percentage points for 1984-2000. I would characterize this as entrenchment of voters, a feature of political polarization. I will be interested to see if the net impact of this year’s conventions is an exception to the trend.
Today the electoral snapshot is at Clinton 312 EV, Trump 226 EV. Margins in many states are quite narrow – note the pastel appearance of the electoral maps in the right-hand column. Consequently the Meta-Margin (how much swing toward Trump would be needed to create an electoral tie) is only 2.5%. This close lead, in PEC’s approach to prediction, makes Clinton’s November win probability only 80%, in a range (20-80%) that I call uncertain. This is our starting point for evaluating the weeks ahead.