In 1964, a survey conducted by the Gallup Poll asked "In general, how much trust and confidence do you have in the wisdom of the American people when it comes to making political decisions--a very great deal, a good deal, not very much, or none at all?" Starting in 1997, the question has occasionally been included in Pew surveys, most recently in March 2016. A similar question, "How much trust and confidence do you have in the wisdom of the American people when it comes to making choices on Election Day?" has appeared several times between 1998 and 2016. The figure below shows average responses (higher numbers mean more trust and confidence), with different colors indicating the different forms:
Confidence in the wisdom of the people was only slightly lower through 2006 than it had been in the 1964, but fell sharply between 2006 and 2015. This pattern is in contrast with confidence in most institutions, which has declined pretty steadily since the 1970s, as discussed in this post (maybe with a bigger drop from the 1960s to the 1970s, as discussed in this one). It would be tempting to say the decline was a response to the rise of Trump, but the average in a survey from September 2015, when the campaign for the Republican nomination was just getting started, and March 2016, when he was moving toward the nomination, were almost exactly the same. So in terms of this question, he seems to have been a symptom more than a cause.
[Date from the Roper Center for Public Opinion Research]