David Gergen is a senior political analyst for CNN and has been a White House adviser to four presidents. A graduate of Harvard Law School, he is a professor of public service and co-director of the Center for Public Leadership at the Harvard Kennedy School. Follow him on Twitter: @david_gergen . The opinions expressed in this commentary are his.

(CNN) Monday night's debate between Hillary Clinton and Donald Trump is shaping up as the climactic moment of this presidential campaign -- one that will re-shape the landscape until Election Day.

If Mrs. Clinton can put away Mr. Trump, she could come close to locking up the presidency, but if he pulls off an upset, the dynamics of the race will likely shift in his direction. All the marbles are on the line here.

I have been engaged in these presidential debates for the past 40 years, first in debate prep for Gerald Ford and Ronald Reagan and since then on television. Like most Americans, I welcome this tradition.

While the formats have never been perfect, these showdowns provide the most revealing insights into the candidates of any conceived. The man who deserves credit for reviving them in 1976, Newton Minow, rightly points out that 78 countries around the world now copy the US debate model in electing their leaders.

Drawing upon polling data, political scientists tend to argue that debates don't really matter in shaping election outcomes but ever since the initial Kennedy-Nixon debates of 1960, I have been among those who beg to differ.

A TV boost for JFK

Jack Kennedy was an underdog in that race, a callow, untested senator challenging a seasoned vice president who had served two terms under one of the most widely admired men in the world, Dwight Eisenhower (over 8 years Ike had an average approval rating of 65%).

Nixon, ever the diligent student (they called him "Iron Butt" in law school), prepared assiduously even as he pressed ahead on the campaign trail. Kennedy relaxed in the sun, studied what he needed, and built up his energy.

Kennedy was also sly: He had a professional make-up artist come to prepare him at home. In the studios, organizers asked if the candidates would like make-up. Kennedy brushed them off, saying he didn't want any. Nixon, his manhood challenged, then swore off too. Guess which one looked like a million dollars on stage -- and which one was wan and sweating.

While scholars dispute whether -- as legend has it -- radio listeners thought Nixon had won, there is no doubt that television watchers made up the vast majority of followers and believed that Kennedy had won. That evening prompted exciting crowds for Kennedy in the days that followed and he charged from behind to win by a whisker. Kennedy later told his press secretary Pierre Salinger that he won the presidency because of television.

Those 1960 presidential debates created a widely accepted view among politicos and relevant to the Clinton-Trump contest: that debates open up more opportunities for challengers than for front-runners and incumbents. President Johnson in 1964 and Richard Nixon in 1968 and 1972 refused debates because they were ahead in the polls.

Debate dangers

Sixteen years passed before an incumbent, President Ford, agreed to debates in 1976 and he did so because he was running way behind his challenger, Jimmy Carter. Ford then proceeded to demonstrate the flip side of the argument that debates can help challengers; he showed how dangerous they can be for a vulnerable incumbent. In the teeth of Soviet domination of Eastern Europe, he insisted that peoples there were free -- a gaffe that turned the campaign decisively in Carter's direction. Ford never recovered.

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.

The good news for Clinton is that front-runners like her have usually done well in debates and have gone on to claim the presidency -- think Reagan in 1984, George H. W. Bush in 1988, Bill Clinton in 1992 and 1996, George W. Bush in 2004, and Barack Obama in both 2008 and 2012. That's 7 out of the past 8 elections. That should give her team some reassurance.

History, then, doesn't hold the ultimate answer. But it is often suggestive, and it suggests that Monday night will be one of the most consequential debates we have ever seen. Hillary goes in as the favorite -- she is far more substantive -- and remains the odds-on-favorite to win in November. Yet, in this most unpredictable of years, Trump has a significant opportunity to change the dynamics of the race. Stay tuned.