The reason, as Morning Consult executive director of polling and data science Kyle Dropp notes, is that many voters appear reluctant to admit to being Trump supporters when talking to another person.
"Republicans are more likely to say they want Donald Trump in the White House if they are taking a poll online versus in a live telephone interview," Dropp wrote. "And, if you're a highly-educated or engaged Republican voter, it turns out that you're far less likely to tell another human being you want Trump as president."
The study identified nearly 2,400 registered Republicans and Republican-leaning independents. All of the respondents initiated the poll online, but only one-third of them completed it online. Another third of them were transitioned to an automated telephone poll, and the final third to a poll conducted by a live interviewer. They were all asked the same questions, at the same time. The only difference was how the questions were presented.
The difference in response rates was pretty stark. Weighting for demographic factors, the study found that Trump got 38 percent of Republican voters who took the poll online, 36 percent of those who took the poll through an automated telephone questioner, and only 32 percent of those who had to talk to another person directly.