A note on party composition in polling samples.
Some commenters have noted that the Democratic advantage in the latest Monmouth University Poll is larger than in our poll taken just prior to the two parties' conventions . Specifically, voters in the current poll self-identify their party leanings as 35% Democrat, 26% Republican, and 39% independent or other. In the July poll it was 33% Democrat, 28% Republican, and 39% independent or other.
Contrary to some misperceptions - largely by those unhappy with the overall results of the latest poll - Monmouth did not "choose" the sample to look this way. Party identification is a self-reported attitude based on where people see themselves fitting in the current political environment.
It is not the same as party registration or partisan voting behavior (e.g. consistently voting in one party's primaries), which is a more stable metric. I wrote about these differences in more detail a few years ago (Party ID Apples and Oranges). While the data in that analysis were drawn from New Jersey voter files and poll samples, the underlying message is the same. Party self-identification can move with the political climate, while party registration is more stable.
Monmouth's 2016 presidential polling uses a combination of voter lists and random digit dialing. The voter list includes data on voter registration and past primary voting. According to this metric, 34% of the Monmouth sample are registered or active Democrats, 34% are Republicans, and 32% are independents or something else.
In other words, the Monmouth sample is evenly divided between Democrats and Republicans when it comes to registration and past voting behavior. Yet when asked how they see themselves politically, these same voters are 9 points more likely to call themselves Democrats rather than Republicans.
The question you should be asking yourself, in light of events over the past few weeks, is why that might be so.