Why tonight can clinch the election

David Gergen | 9/26/2016, 3:30 p.m.
Monday night's debate between Hillary Clinton and Donald Trump is shaping up as the climactic moment of this presidential campaign ...
Donald Trump says the swirling controversy over Hillary Clinton's email will cause her to drop out of the 2016 presidential race. (Photo: CNN)

— If there was anyone who proved just how helpful debates can be for a challenger and how perilous they can be for an incumbent front-runner, it was Ronald Reagan. I was recruited to be part of his debate team as he took on President Carter in 1980. I didn't know him well at the time and was impressed how hard he worked to prepare.

Walking in, he had a humorous one-liner ready to use against incoming attacks, and he closed with a rhetorical question that was quoted long after: "Ask yourself if you are better off than you were four years ago?" Reagan seized control of the race that night and never looked back.

Yet four years later, when he was the oldest man ever to serve in the presidency, the Gipper often seemed befuddled in his first debate with challenger Walter Mondale. Even though he had been far ahead in the polls, Reagan's campaign suddenly seemed in peril under an onslaught of stories asking whether he was too old for the job. His sense of humor turned things around in a second debate when he promised he would never exploit the youth and inexperience of his opponent. Mondale said he instantly knew the race was over.

And so it has gone in the many debates over the years: Challengers sometimes seize upon debates to catapult up while front-runners and incumbents sometimes stumble and are pounced upon by their opponents and the media. In the first debate four years ago between President Obama and Mitt Romney, Obama was listless and unfocused and Romney was energetic and on target, winning the night. An incumbent front-runner once again showed how easy it is to stumble. Had he not roared back in the second debate, Obama might have been in trouble.

Good news for both sides

So, what does all this history tell us about Trump and Clinton Monday night? There is good news for both sides -- but lots of cautions, too.

The good news for Trump is that challengers can upset a vulnerable front-runner through a strong first debate, change the dynamics of the campaign and go on to victory in November -- think Kennedy in 1960 and Reagan in 1980.

Hillary Clinton's team is right to be concerned about whether Trump will have a lower bar to clear than she will. By 53-43%, voters in a CNN poll believe she will win the debate. With expectations so much lower for him, Trump doesn't have to beat her to improve his standing; he just has to tie her.

He only has to show that he is presidential, too -- that he would have the capacity, judgment and character to take on unexpected crises as well as the daunting problems that go with the job. He doesn't have to show that he has a mastery of every issue but that he has a clear sense of direction and a pretty decent chance of succeeding in office.

To win, Hillary Clinton must not only prove that she is presidential but she must also show that she is more likable. Voters often ask themselves two questions: (1) Will this candidate be a safe, reliable person in the Oval Office? (2). Will I be happy having this candidate in my living room every night for the next four years? She is winning on #1 but struggling on #2. On Monday night she needs to move the needle on #2 -- and avoid the gaffes that have plagued some front-runners.