It may seem like I'm beating a dead horse here, but I'm not. The night mare is very much alive.
Today Reuters/Ipsos formally released the results of its latest five-day tracking poll headlined "Clinton extends lead over Trump to 13 points". The national poll of likely general election voters, which was administered entirely online, was made up of a sample that was 50.7% Democrat, 36.3% Republican, and 13.0% independent or third-party.
That's wildly out of line not only with other measures of partisan affiliation in the US--in late June Gallup found a 31%-28%-39% distribution, respectively--but also with the polling samples of Reuters/Ipsos' polls just a couple of months ago, when Democrats made up around 45% of those surveyed and Republicans around 42% (and when Trump briefly led).
What's particularly remarkable is that there is no mention of the partisan distribution of the sample anywhere in the write up. It can be found at the source but not in the media release.
The change in the political makeup of the polling sample accounts for virtually all of the gains Hillary (appears) to have made against Trump since May. Hillary's support among independents and third-party voters has actually declined over the last couple of months while her support among Republicans has stayed constant at around 7%, even as her overall support in the R/I poll has increased.
Reuters/Ipsos is an outlier in the current RCP average. While it shows Hillary leading Trump by 13 points, her average lead across the other seven polls currently displayed is just 3.6 points. The most recent polls released in Iowa (today) and Florida (yesterday) actually show Trump ahead in both, and these are states Obama won in 2012.
Something feels fishy. To reiterate, it could be a consequence of Republican recalcitrance in the face of Trump's refusal to morph into John Kasich, the tendency for people to identify with whatever party the candidate they intend to vote for is from, a problem with Reuters-Ipsos' sampling methodology (either inadvertently, or, more sinisterly, intentionally), some combination of these things, or something else entirely.
Then again, maybe I'm not being cynical enough. Perhaps Heartiste, who is nothing if not perspicacious, offers a better explanation: