1. "Tuned in" means they watched on TV. Your example person need not be physically in Miami on the 9th of March.
2. The word "appall" is seldom used. It is most often seen as appalled as in the Economist article.
2. The fact of John being a moderate Republican does not by definition mean he is appalled by Trump. It's possible to imagine moderate Republicans who might even support him.
2. Donald Trump, as of the time of the article in question (and this writing), has not yet been nominated. A strong case can be made for being appalled by Donald Trump even if he were not running for President.
Here's my take on the article:
A. Assume John is a moderate Republican.
B. He does not like Donald Trump. In fact, he is appalled by the possibility of him becoming President.
C. Maybe he doesn't like any of the other Republican candidates, either.
D. Moderate Republicans may hate Trump so much that, if he did become the Republican candidate, they would vote for the Democratic candidate (presumably Hillary Clinton) instead; just this one time.
E. John watched the Democratic debate on television that was held in Miami on March 9th.
F. Things that Hillary Clinton said during that debate might cause John to consider some third alternative.
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