James Fallows has a long article in the upcoming Atlantic attempting to answer this question:
The most famous story about modern presidential campaigning now has a quaint old-world tone. It’s about the showdown between Richard Nixon and John F. Kennedy in the first debate of their 1960 campaign, which was also the very first nationally televised general-election debate in the United States.
The story is that Kennedy looked great, which is true, and Nixon looked terrible, which is also true—and that this visual difference had an unexpected electoral effect. As Theodore H. White described it in his hugely influential book The Making of the President 1960, which has set the model for campaign coverage ever since, “sample surveys” after the debate found that people who had only heardKennedy and Nixon talking, over the radio, thought that the debate had been a tie. But those who saw the two men on television were much more likely to think that Kennedy—handsome, tanned, non-sweaty, poised—had won.
Historians who have followed up on this story haven’t found data to back up White’s sight-versus-sound discovery. But from a modern perspective, the only surprising thing about his findings is that they came as a surprise. Today’s electorate has decades of televised politics behind it, from which one assumption is that of course images, and their emotional power, usually matter more than words and whatever logic they might try to convey.
Never has the dominance of the image over the word seemed more significant than this year, as the parties and the public prepare for the three general-election debates between Hillary Clinton and Donald Trump that are scheduled to begin September 26 (as it happens, the anniversary of that first Kennedy-Nixon debate) and the one vice-presidential debate between Tim Kaine and Mike Pence, scheduled for October 4.
The whole thing is worth a read. I'm frustrated that I'll be in a rehearsal during the first debate, but I may stay up late after watching it.