From David Zarefsky, professor emeritus of communications studies at Northwestern University:
Vice-presidential candidates are in an interesting position. They largely act as surrogates for the person at the top of the ticket, but they also usually try to establish their own suitability to be president if they should be thrust into the role. One might have expected Pence and Kaine to receive more than the usual amount of scrutiny as possible presidents because of the age of their running mates. But so far that hasn’t happened, possibly because Clinton and Trump are close to the same age. The vice-presidential debate, and for that matter the vice-presidential race, usually makes no difference in the election. The one possible exception in recent years is the 2008 contest between Joe Biden and Sarah Palin. Vice-presidential debates do produce some memorable lines, such as Bob Dole’s reference to “Democrat wars” in 1976, Lloyd Bentsen’s put-down of Dan Quayle, “You’re no Jack Kennedy” in 1988, and Admiral James Stockdale’s opening line, “Who am I and why am I here?” in his three-way debate with Dan Quayle and Al Gore in 1992.