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At last, the moment has arrived: Donald Trump and Hillary Clinton go mano a mano Monday night in their first presidential debate.

Beyond gender, the contrasts between the two could not be more stark. Trump is a celebrity billionaire and novice politician, light on details, strong on bravado. Clinton is a seasoned public servant 鈥� a former first lady, United States senator, and secretary of State 鈥� and steeped in 10-point plans.

Trump has never gone one-on-one in a political debate before, while Clinton has appeared in 40 debates overall during her political career. So Clinton should 鈥渨in鈥� on Monday, right? Not necessarily. Voters are clamoring for change, and if Trump can convince enough voters that he鈥檚 presidential material, he could build on his momentum. Polls now show the two in a dead heat.

More than 100 million people are expected to watch worldwide 鈥� a record for a presidential debate. It will run 90 minutes, from 9:00 p.m. to 10:30 p.m. Eastern Time, and take place at Hofstra University on Long Island. The themes, selected by host Lester Holt of NBC, center on the economy and security. Here鈥檚 everything else you need to know:

How important is this debate, really?

Quite important, actually. It鈥檚 a rare opportunity for American voters to see the two main candidates in an unfiltered and unscripted way, interacting with each other and the moderator, and responding to questions. Voters can size up the candidates, and try to imagine each sitting in the Oval Office.

Whether this debate and the other two (Oct. 9 and 19) could actually decide the election is another question. 鈥淒ebates can move the polls but rarely determine the winner of the election,鈥� write John Sides and Lynn Vavreck in their book 鈥淭he Gamble.鈥�

But this year could prove to be an exception, given the presence of Trump, an unorthodox candidate who faces questions over his temperament, and Clinton, long in the public eye but facing major concerns over trustworthiness. They are the least popular major-party nominees in history, and Monday鈥檚 debate could be their single best opportunity to allay concerns.

鈥淔or a lot of voters, this will be the first real impression of either candidate,鈥� says Republican strategist Ford O鈥機onnell. 鈥淪o there鈥檚 a lot riding on this debate.鈥�

Could it be the biggest since the first Nixon-Kennedy debate in 1960?

Yes. There have been many memorable debate moments in modern history. In 1976, President Ford inexplicably claimed 鈥渘o Soviet domination of Eastern Europe." In 1980, former California Gov. Ronald Reagan nailed President Carter with 鈥渢here you go again.鈥� In 2000, Vice President Al Gore was caught sighing loudly while Texas Gov. George W. Bush spoke.

But none of those moments are seen as pivotal to the eventual outcome of those races. The election of 1960 is a different story. The first debate between Vice President Richard Nixon and Sen. John F. Kennedy 鈥� 56 years ago today 鈥� featured a pasty, sweating Vice President Nixon and a tan, smiling Senator Kennedy.聽 (You can watch a video of it .)

It was the first televised presidential debate in US history, and gave Kennedy a boost in the polls 鈥� an edge that historians say was pivotal to his narrow victory in November.

What does Trump need to do to succeed?

Trump鈥檚 performance during the GOP primary debates was erratic 鈥� but ultimately effective. He slung insults (鈥淟yin鈥� Ted,鈥� 鈥淟ittle Marco鈥�) at his opponents, knocking them off-balance but also earning a reputation as a bully. His rhetoric tended toward stump-speech slogans, and was light on policy details, but as a showman, he stood out from the crowd in most. (One debate he skipped.) He was also aggressive toward the moderators.

Now the ballgame has changed. His goal Monday is to appear presidential, and not necessarily entertaining. Voters will be watching for tone and demeanor as much as substance, Republicans say.

Trump 鈥渏ust has to be presidential in the three debates鈥� and he will win in November, says former Gov. John Sununu (R) of New Hampshire. 鈥淭hat鈥檚 what this election now comes down to: three nights of self-discipline.鈥�

Trump himself has promised to be 鈥渧ery respectful鈥� of Clinton. 鈥淚 think she deserves that,鈥� he said on Fox News last week. 鈥淎nd I鈥檓 going to be nice.鈥�

Style points will matter more than what鈥檚 actually said, says Mr. O鈥機onnell. 鈥淭here will be a couple of one-liners that everyone will focus on, but so what?鈥� he says. 鈥淗is goal is very simple: If you can see him in the Oval afterwards, then he鈥檚 done his job. He doesn鈥檛 have to do better than Clinton, he just has to be plausible.鈥�

What does Clinton need to do to succeed?

Clinton鈥檚 challenge is different from Trump鈥檚. She鈥檚 a strong debater in arguing points of policy, but needs to reveal more of herself to voters and project warmth, analysts say. 聽On controversial matters, such as her private email server and her health, she needs to respond in as straightforward a manner as possible. Both candidates face challenges with voters on honesty and trustworthiness, but Clinton than Trump.

Clinton spokeswoman Jennifer Palmieri has said that Clinton was 鈥減reparing for the different Trumps that might show up,鈥� referring to his repertoire of behaviors.

Clinton herself told radio host Steve Harvey last week that she was going to do her 鈥渧ery best to communicate as clearly and fearlessly as I can in the face of the insults and the attacks and the bullying and the bigotry that we have seen coming from my opponent."

But if Trump comes across as staid and dignified, and skips the insults, she will have to play a different game. Democrats are concerned that the bar for Trump is low, and that just behaving in a minimally dignified way will give him a boost. Clinton鈥檚 task is trickier.

鈥淪he鈥檚 a woman with tremendous experience but more than that, she鈥檚 got a real heart,鈥� says Democratic strategist Peter Fenn. 鈥淏ut who knows what the heck people are going to come out of this with.鈥�

Mr. Fenn predicts that between Holt and Clinton, questions that Trump isn鈥檛 prepared for are bound to come up 鈥� and that will test him.

鈥淚f they decide to do any amount of gotcha stuff, he鈥檚 going to have a heck of a time,鈥� says Fenn. 鈥淭his is not his strength.鈥�

And that could give Clinton a boost.

What happened to Gary Johnson and Jill Stein?

Mr. Johnson, the nominee of the Libertarian Party, and Ms. Stein, nominee of the Green Party, did not qualify for the debate. Under the rules of the Commission on Presidential Debates, which is controlled by the Republican and Democratic parties, a candidate must receive at least 15 percent of the vote in five national polls to qualify. They failed to meet that criterion.