The key to winning televised presidential debates, and ultimately the election, is for a candidate to come across as being the "aggressor," rather than being "aggressive." It's an important distinction.
The "aggressor" is the candidate who answers questions with forceful language and a laser focus that takes advantage of both the mood of the moment and the opponent's hesitancy and faulty logic. The "aggressive" candidate, by contrast, tries too hard and comes across as negative, petty or defensive.
John Kennedy indeed was more telegenic than Richard Nixon in the 1960 debates. But he bested Nixon also because he brilliantly hammered home the theme of "change," much as Barack Obama did in 2008. Kennedy was the aggressor.
The next televised presidential debates were in 1976. Jimmy Carter and President Gerald Ford were both aggressors. But Ford blundered badly by saying that Eastern Europe wasn't under the domination of the Soviet Union. Many Americans saw this as evidence that he didn't have command of the facts.
The tables turned in 1980. Ronald Reagan was the aggressor against Carter. Reagan combined a deft handling of the facts with timely humor. ("There you go again!" said Reagan, famously.)
Oddly, it was Reagan who came across as weak in his first debate against Walter Mondale four years later. That got the press speculating whether Reagan was getting too old to remain president. But Reagan rescued himself in the next debate. His knockout blow was a funny line about how he wouldn't hold Mondale's youth and inexperience against him.
Now consider the 1988 debates between Massachusetts Gov. Michael Dukakis and Vice President George H.W. Bush. Dukakis had already taken a pummeling in a televised ad that revealed that he had ordered the prison release of convicted killer Willie Horton. Horton got out and then raped someone.
Dukakis made things worse in a debate. When asked what he would do if someone raped his own wife, Dukakis answered with a lukewarm defense of why the death penalty fails to deter crime. His answer appeared to show no personal concern for his wife. He lost the election.