And although the unpredictable has a tendency to become the headline, the issues and questions hovering over this debate have been calcifying for weeks. Here are five things to watch.
1. Trump's attacks
For the past month, Trump has had the longtime anti-Clinton investigator David Bossie at his side, providing him with reams of information about past controversies. And during that period, Trump has mostly focused his Clinton critique on her record in office and inability to fix what he considers the country's chronically broken political and economic system. But more than once, he has delved into the tabloid aspects of Clinton history, such as when he tweeted tauntingly Saturday about inviting Gennifer Flowers, who said she had an affair with Bill Clinton, to the debate.
Monday night will provide a window into the extent to which Bossie is influencing Trump and whether he has internalized his adviser’s opposition research about 1990s-era scandals and more recent issues. Can Trump pack together points on Clinton’s paid speeches to Wall Street firms and her bygone advocacy for trade deals into a populist rebuke? Or does he throw around terms such as “Benghazi” and “her emails” with flair but little elucidation?
For Trump, weaving together the many threads on the Clintons’ time in the White House and after into an argument that is both intelligible and damning won’t be a natural undertaking for a candidate who relishes extemporaneous zingers and confrontation. But it may be necessary for success against the polished Clinton, who has swatted back Republican criticism for decades.
The risks and rewards for Trump are plenty. If he can calmly frame Clinton as untrustworthy and an agent of the financial and political establishments, he could leave the stage having given some voters pause about her character and associations. If he is scattered and conspiratorial, and neglects to use his most potent material, he could leave voters wondering more about his coherence than her record.
Trump’s innate understanding of what makes for electrifying political theater was crucial to his success during the crowded Republican primary debates. On Monday, he will be on a far more subdued stage, staring back at one opponent, and with the question of his temperament at the fore.
Viewers will be evaluating whether he seems “presidential,” that enigmatic blend of qualities befitting a statesman, chief executive and commander in chief. Even those who chuckled at his quips or cheered him in the primaries may now wonder whether he should be entrusted with the nuclear codes.
How Trump handles the setting — and especially how he responds to Clinton's likely provocations — could shape the consensus on his performance. Will he become distracted and furious if Clinton supporter Mark Cuban, a billionaire Trump frenemy, is smirking in the front row? Will he lose his cool if Clinton baits him into an exchange about his net worth, his charitable foundation, his bankruptcies or Trump University?
A flurry of red-faced outbursts that veer into the awkward or the strange could doom him. A night where he balances crackling showmanship and a reassuring disposition could lead to gains.
Clinton’s temperament is less of a factor because she is so well known as a seasoned and measured political actor. But her reactions to Trump’s shots will be a significant test. Being dismissive about personal comments about her stamina or Bill Clinton’s affairs, for instance, could win her sympathy from voters who consider him out of bounds. Dismiss him entirely, however, and she risks her incredulity being seen as aloofness, as Al Gore’s famous yawning sighs about George W. Bush became a touchstone in 2000.
3. Clinton's vulnerabilities
Ask undecided voters what they think of Clinton and they use words such as corrupt and untrustworthy, uninspiring and disingenuous. On Monday, she not only has to convince these voters that Trump is unfit to be president, but also that she would be a president they could support and admire.
Clinton’s team considers this high-stakes moment her best chance to do so, not so much by addressing her vulnerabilities directly but by selling her attributes and accomplishments. For the past month, she has laid the groundwork for this approach, trying to raise her standing on questions such as trustworthiness and likeability in several speeches.
Still, one variable will be whether Clinton shows any contrition on the issue of her private email server as secretary of state. That controversy has dogged Clinton’s campaign for more than a year, and her defensive, strained and lawyerly handling has fed the impression that she is covering something up or hiding the complete story.
How Clinton handles questions about her emails from the moderator and, perhaps more important, forceful allegations of wrongdoing from Trump could be critical to her cause. Will she sound overly defensive? Or will she own up to some mistakes and find a way to diffuse the issue?
More broadly, Clinton backers call her disciplined but some voters do view her as overly scripted, in contrast to the more freewheeling Trump. And those voters may be watching to see whether she comes across as authentic and capable of spontaneity, even if her supporters say such issues are frivolous or sexist.
If her recent interview with comedian Zach Galifianakis on his “Between Two Ferns” online series is any indication, Clinton will strive Monday to show some levity, without going too far.
Trump’s campaign has skewered the normal partisan boundaries of modern presidential politics, putting a New York real-estate mogul and Democrat-turned-Republican atop the GOP ticket with a pitch that strays from party orthodoxy. Unlike many Republican leaders, Trump isn’t inclined toward a hawkish worldview and he wants to preserve federal entitlement programs such as Medicare and opposes free trade.
Trump’s challenge will be articulating his jumbled ideology and describing his positions as an overture to voters in the center and on the left who are frustrated with both political parties. If he is unable to do so, he risks being defined solely on Clinton’s terms and as a hard-liner on immigration who speaks in incendiary ways about race and ethnicity. That would be a lost opportunity for Trump's populism-infused campaign.
Clinton’s ideological dance will be similarly complicated, although her challenge will be less about defining herself to the electorate at large than in rousing liberal voters who remain skeptical, notably young voters. Without lurching so far to the left that she loses her appeal to moderates, Clinton will have to convince the backers of Sen. Bernie Sanders (Vt.) and those tempted by third-party candidates that she embodies their desire for advancements on social and economic policy.
Although Trump’s temperament has been Clinton’s main target, she may try to fully connect him with the Republican Party and all that entails ideologically, linking him with the unpopular congressional GOP agenda to restrict abortion rights and overhaul Social Security, as a means of erasing his self-portrait as something different from them.
5. The moderator
Who is responsible for correcting falsehoods and exaggerations, the debate moderator, Lester Holt, or the opposing candidate? This is one of the night’s looming questions, and both the Clinton and Trump campaigns, and their supporters, are strongly advocating different approaches that are shaping early perceptions.
Trump’s camp says the moderator should simply ask questions; Clinton’s says the moderator has a responsibility to interject and educate the audience. Her advisers say she will be prepared to call out any lie by Trump, although she’d rather see Holt intervene.
Speaking Sunday on ABC’s “This Week,” Clinton campaign manager Robby Mook said it would be unfair for Clinton to have to play “traffic cop” as she makes her case to voters. Trump campaign manager Kellyanne Conway, also on ABC, dismissed “these virtual fact-checkers” at news organizations and said moderators should not do the bidding of campaigns.
According to the Commission on Presidential Debates, the nonpartisan host, moderators are supposed to ask questions to spur discussion, but the onus falls on the candidates to correct each other.
Perception will be everything in the expected disputes over what Holt, the host of NBC’s “Nightly News," does (or doesn’t do). This will be particularly true if Trump, who routinely makes inaccurate or misleading assertions, is seen as advancing his candidacy without being called on a particularly glaring statement, or if Clinton is seen escaping scrutiny or being overly subject to it.
The country’s eyes will be on Holt and whether he can deftly navigate the sparring, all while not appearing too hot or too cold in his willingness to step in.