Why Were Leftists Able to Describe the Trump Movement Accurately?
My theory is that leftists were able to describe the Trump movement accurately because their description was an accurate description of right-wing Democrats. It had little to do with conservatives in general, partly because right-wing Democrats are conservative on only a handful of issues.
Over the past generation or two, right-wing Democrats have been joining the Republicans. That accelerated over the past eight years. As a result, they formed a critical mass this year, which exploded and was able to take over the Republicans. They were helped by the fact they were united behind one candidate while traditional conservatives were debating the relative merits of Senators and Governors or the relative importance of electability vs. purity.
If Hillary is elected but Trump comes close, she might take steps to bring them back. If Trump is elected and ignores his base, they may get discouraged next time. In either case, in 2020 or 2024, we will probably see more than one would-be Trump. That may keep them from taking over again.