The first presidential debate of the general election is often the most treacherous - especially for the candidate who steps on stage with the presumed advantage.
Which is why Democratic nominee Hillary Clinton, the one in that position this time around, knows not to take anything for granted.
"You can't really win an election in a debate, but you can lose one," said Brett O'Donnell, a communications consultant with long experience coaching Republican presidential candidates. "The first debate is the most important of all the debates, and it definitely has the most potential to harm."
Examples of first-debate stumbles are many. And they have almost always hurt the candidate for whom the expectations were higher.
The biggest pitfall is a blunder that confirms the misgivings that voters may already be harbouring.
A confused Ronald Reagan rambled in 1984, opening doubts about whether he had become too old to do the most important job in the world.
In 2000, Al Gore sighed and exaggerated. George W. Bush casually draped himself over the lectern in 2004 and peevishly quibbled on minor points. Four years ago, an aloof Barack Obama seemed to phone it in.
Clinton is seen as the nominee best equipped to present herself as the more credible and appealing potential president.
A CNN/ORC poll this month found that 53 per cent of respondents think the former senator and Secretary of State will do a better job in the debate, while 43 per cent said that of GOP nominee Donald Trump, the real estate developer and reality TV celebrity.
That has Clinton's team arguing that Trump should not be graded on a curve.
The Republican nominee "should be held to the same standard on knowledge - what kind of plans you have, your ability to explain your plans, skills, expertise you have," said Jennifer Palmieri, Clinton's communications director.
The biggest unknown for Clinton and her strategists is which version of Trump will show up.
It will be the first time he has appeared on a debate stage with only one other candidate, which means the spotlight will be harsher and more intense. It also represents a test of how well he can pitch his message to an electorate broader than Republican primary voters.
Trump has indicated that he will approach the debate as he has pretty much everything else in his campaign - reactively, and by trusting his own instincts and impulses.
"People ask me that question, 'Oh, you're going to go out there and do this and that.' I really don't know that," Trump told Fox News Channel's Bill O'Reilly. "You're going to have to feel it out when you're out there. She's got to treat me with respect. I'm going to treat her with respect. I'd like to start off by saying that, because that would be my intention."
Will Trump be the aggressive, name-calling combatant who dominated the crowded GOP field, or will he take a more measured, statesmanlike posture?
Trump is capable of doing the latter, as he showed during the primary season. His best moment might have come when Senator Ted Cruz attacked Trump's "New York values," and the real estate developer responded with a moving account of the character that his city showed after the September 11, 2001, terrorist attacks.
"It came from the heart. It was a good attack, and it wasn't over the top," O'Donnell said. "If that Donald Trump shows up, Hillary Clinton is in trouble."
Figuring out Trump's moves is "hard to game out," Palmieri said. So Clinton may be better served by focusing on what she can control, which is fashioning her own argument.
Indeed, modern history shows that candidates - especially in the first debate - are often most hurt by the damage they do to themselves.
Reagan vs Mondale
When Ronald Reagan appeared confused and muddled in his first outing against former Vice-President Walter Mondale in 1984, it reminded voters that, at 73, Reagan was the oldest commander in chief in the nation's history.
Reagan tried to reprise a line that he had used to great effect in his only 1980 debate against Jimmy Carter - "there you go again" - only to have Mondale throw it back at him with a reminder that Reagan had tried to cut Medicare after saying he would not.
The President later blamed his performance on the fact that his advisers had attempted to compensate for another perceived deficiency, a lack of policy depth, by cramming his mind with minutiae during debate prep.
Mondale's own internal polling showed that his favourability went up 23 points that night, said Democratic strategist Tad Devine, who was working for the Democratic nominee's campaign.
But Reagan put the age issue to rest with a single quip in the second debate, when moderator Henry Trewhitt of the Baltimore Sun asked bluntly whether the septuagenarian had any doubt that he would be "able to function" in a national security crisis.
"Not at all, Mr Trewhitt, and I want you to know that also I will not make age an issue of this campaign," Reagan replied. "I am not going to exploit for political purposes my opponent's youth and inexperience." Reagan swept 49 states in the election.
Gore vs Bush
In the pre-Twitter era of the 2000 election, Gore's strategists thought that the Vice-President had won the debate against Texas Governor George W. Bush - as evidenced in an instant poll by Gallup - until the reviews and the fact-checkers caught up with him the next day.
Gore had been such a stickler for debate preparation that he was known for insisting that the thermostats in his rehearsal room be set to the exact temperature as those in the debate hall, with an adjustment for the body heat of the crowd.
But those were not the atmospherics he should have worried about. When the real event happened in Boston, the microphones caught Gore emitting a heavy sigh when Bush chastised him for "fuzzy math". Gore also embellished some details in his accounts of school overcrowding and the role he had played in disaster relief.
It reinforced doubts about Gore's personality and honesty.
"Part of our problem as the staff was we didn't recognise it as a big error," said Devine, who was a top adviser to the campaign. "As it turned out, it fed into a story line that was very bad for him."
To impress upon Gore how badly he had done, and how deeply the impression had seeped into popular culture, his staff had him watch a lampoon of his performance on Saturday Night Live.
Gore never fully recovered in the subsequent debates. The final one included a peculiar moment when the Vice-President strolled over to Bush to demand his position on a piece of health-care legislation - getting a glare and contemptuous nod in return.
Bush vs Kerry
Four years later, however, it was Bush who was put on the defensive, on the subject of foreign policy, which was thought to be his strength.
Democratic nominee John Kerry attacked him for failing to enlist a broad coalition of allies before going into Iraq - ticking off only Great Britain and Australia. Bush shot back: "Actually, you forgot Poland".
That rebuttal became a running joke with his detractors.
Obama vs Romney
Because sitting presidents rarely face serious competition in their party primaries, they often arrive at their first debate out of practice, overconfident and impatient.
That was the case with Bush in 2004 and Obama in 2012.
Obama "looked like someone had slipped him an Ambien," Politico's Roger Simon wrote. "It's not that Romney's performance was perfect or polished - it wasn't - it's just that Obama's was so mediocre."
Most frustrating to Obama's allies was the fact that he let slip by one opportunity after another to attack Romney for his wealth and business practices. But Obama found his footing again in their next encounter.
Tomorrow, however, will be a different dynamic - given that the two contenders have higher negatives in the polls than any major-party contenders.
So in addition to facing each other, Clinton and Trump will be trying to allay the doubts that voters have about them.
Clinton's centre on her character, and whether a fixture of the establishment can deliver the change that a restive country is demanding. Trump's are about his temperament, and whether he has the policy depth to be a credible president.
"I think this debate is even more important, because there are so many basic unanswered questions about these candidates that, by now, we normally have answers to," said O'Donnell. "There's some fundamental things that people will be interested in that will make these the most-watched debates - especially the first one."