Donald Trump exudes confidence heading into Monday's first presidential debate. Yet, history suggests that he is hurtling toward disaster unless he exchanges his brawling style for warmth and empathy.

Once presidential campaigns transformed into a contest of personalities and conventions morphed into candidate infomercials, debates became the best opportunity for voters to peer into the candidates' souls. Often, their takeaway has little to do with what candidates say and everything to do with likability and connecting on a personal level. A good impression can shatter caricatures generated during the campaign. But one mangled sentence, one unguarded facial expression, or one ill-conceived taunt can alienate voters and reinforce negative impressions.

Richard Nixon and John F. Kennedy introduced modern-day Americans to presidential debates in 1960. Though historians have debunked the myth that Nixon's pallor and sweating lost him the first debate in the eyes of television viewers, he sensed that the debates contributed to his razor-thin loss.

The next presidential debate occurred in 1976. President Gerald Ford was trailing badly in the polls and challenger Jimmy Carter needed to convince voters that he possessed the capacity and knowledge to serve as president. In their second encounter, Ford demonstrated why front-runners avoided debates between 1960 and 1976. He wounded himself by declaring "there is no Soviet domination of Eastern Europe and there never will be under a Ford administration." This gaffe reinforced the perception, popularized by Saturday Night Live, that the unelected Ford was a witless bumbler in over his head.

In 1980, Carter only squared off with Ronald Reagan once, but it was enough to reassure voters who yearned for change that Reagan wasn't the trigger-happy extremist painted by Carter's campaign.

Eight years later, Michael Dukakis unwittingly reinforced George H.W. Bush's caricature of him as an iceman who cared more about legal niceties than right and wrong. Infamously, Dukakis responded to a question about whether he would support the death penalty if his wife was raped and murdered by bloodlessly outlining his opposition to capital punishment instead of offering the vengeful rant invited by the question.

Four years later, Bush, who trailed in the polls, labored to overcome perceptions that he could not understand the economic struggles of average Americans during a recession. During the first presidential town hall debate, television cameras spied him glancing at his watch, which reinforced the impression that he was aloof, out of touch, and resigned to defeat.

In the debate's defining moment, Bush fumbled a question from a voter about how the national debt personally affected him, appearing both befuddled and defensive. He asserted that people ought to be in the White House with him, see what he saw, and read the mail he read before pleading, "I don't think it's fair to say, 'You haven't had cancer. Therefore, you don't know what it's like.'" In contrast, Bill Clinton strode over to the questioner and empathetically discussed knowing people who lost jobs or whose factories closed in his small state.

Al Gore failed to learn from Clinton's example. In his first showdown with George W. Bush in 2000, Gore alienated voters by repeatedly sighing, and grimacing - which the media and Bush's campaign pounced on after the debate. Though Gore won the overnight postdebate polls, and generally impressed voters with his policy knowledge, his performance reinforced the impression that he was supercilious, condescending, and unlikable.

This history tells us that, unless a candidate epically stumbles, the substance of their answers is likely to matter less than their tone, behavior, and likability in determining who wins or loses the debates. This bodes poorly for Trump.

During the campaign, Trump has demonstrated that he'll lash out against anyone who challenges him. He also regularly deploys caustic nicknames and mockery, while launching frequent bromides. Empathy, warmth, and humility seem absent from his playbook. This style boosted him in multicandidate primary debates, where fans cheered his no-holds-barred approach and he distinguished himself from the pack.

But the history of general election debates suggests that this style - pugnacious enough to attack Gold Star parents, a respected federal judge, and a disabled reporter - is likely to repel voters who crave change but doubt that Trump possesses the temperament to be president.

To win, Trump would need to mimic Reagan's approach in 1980. Reagan wielded warmth and gentle chiding to vanquish Carter, parrying an attack with the famous quip, "There you go again." Even when he asked if Americans were "better off today than you were four years ago," Reagan seemed more mournful than caustic. He certainly didn't call Carter "Failed Jimmy." Likewise, Clinton's ability to "feel the pain" of economically depressed voters in 1992 propelled him to victory.

If, however, Trump sticks to his guns, he risks following the path to defeat trod by Dukakis, Bush, and Gore, all of whom soured debate viewers with their tone and demeanor.

Brian Rosenwald is an alumni research and service fellow at the University of Pennsylvania's Robert A. Fox Leadership Program.