Monday night’s 90-minute meeting between Donald Trump and Hillary Clinton is expected to be the most watched ever, with an estimated 100 million Americans viewing it on their televisions, computers, and smartphones.

There are few comparisons — President Jimmy Carter and Ronald Reagan in 1980, or Sarah Palin and Joe Biden for vice president in 2008 — and they are barely in the same league.

The first presidential debate of 2016 may also be the most anticipated in US history.

The reason is simple: No one has ever seen a presidential debate like this before. Here are four ways it will make history:


1. It’s unpredictable.

Most presidential debates fit within the framework of the zeitgeist of the campaign and past debates. You can anticipate the topics and candidate positions on certain issues. The unknowns are what the candidates will wear and whether they have any special zingers prepared.

“Of all the presidential debates in the past, this one is the hardest to get a handle on in advance,” said Alan Schroeder, a Northeastern University journalism professor and author of “Presidential Debates: Risky Business on the Campaign Trail.” “Part of this is that Trump is so inconsistent and could say anything.”

Sure, the debate topics are baked into the format: jobs, terrorism, foreign policy, health care, trade, and racism. But the traditional left-versus-right dynamic is flipped: Trump is to the left of Clinton on trade and military interventionism, for example.

Also adding to the unpredictable nature of the debate: There is no consensus on which candidate has to make up more ground in the campaign or has the most to lose.

Heading into the first debate in 2012, Republican nominee Mitt Romney had to make up some ground after his 47 percent comment wounded him badly in the polls. The first debate helped to make the race with President Obama competitive again.


But in the days before the first debate of 2016, it is difficult to assess which candidate has to be on the attack. Trump has been gaining in polls both nationally and in swing states, but Electoral College math suggests Clinton is the overwhelming favorite.

Does Clinton look more presidential because her answers are likely more practiced? Or does Trump, just by being on the presidential debate stage, automatically appear more presidential?

Finally, like every presidential debate, it is live on television. Like sports games or the Academy Awards, anything can happen, which only adds to the anticipation.

2. It’s Trump.

He’s an unprecedented force on the debate stage. Media analysts credit him for breaking ratings in the Republican primary debates.

So far, it appears like he’s preparing for this debate like he did for those earlier this year. The upcoming schedule for both candidates suggests that while Clinton has taken days off the campaign trail to prepare, Trump has not reserved much time aside from a few meetings and the final weekend.

So will Trump tell Clinton that she, and the nation’s political elite should be fired? Will he call her “Crooked Hillary?” to her face?

“I know what Donald Trump should do in the debate, but that doesn’t mean that is what he wants to do,” said Stuart Stevens, Romney’s 2012 strategist. “He needs to come across as likable, solidify Republican support, and he needs to say he has made mistakes and be specific about which ones.


“But who knows what his game plan is,” Stevens said.

3. There’s a man and a woman on stage.

Don’t overlook the history made in the first seconds of the debate. When Clinton takes the stage, it will be the first time a woman will be in a general election presidential debate for major parties.

In other debates, the gender dynamic has proved problematic for the male politician — especially when it comes to attacking the female opponent.

In 2000, for example, Clinton’s Republican opponent for US Senate, Rick Lazio, suffered political damage after he aggressively approached her podium during the debate and asked her to sign a contract that banned soft money in the race.

Trump, himself, was rebuked by Carly Fiorina in a debate earlier this year when he said she had “a beautiful face.”

4. Both candidates are deeply disliked.

Trump and Clinton are the two most unlikable presidential candidates in the history of US polling. The most recent Reuters/Ipsos national poll said that 58 percent of likely voters didn’t like Trump, while 55 percent didn’t like Clinton.

Ahead of their first simultaneous television appearance, both candidates will also have to make a decision about whether they want to come across as more likable or simply prosecute their opponent. If their respective national conventions were any indication, then this could be a particularly nasty debate.

It could also make for compelling television.

James Pindell can be reached at james.pindell@globe.com. Follow him on Twitter @jamespindell or subscribe to his daily e-mail update on the 2016 campaign at www.bostonglobe.com/groundgame.