Millions of Americans will be riveted to television and computer screens tonight for the first face-to-face encounter between two presidential candidates whose political philosophies could not be more diametrically opposed.

Democrat Hillary Clinton and Republican Donald Trump square-off in the first of three scheduled debates in what many have described as an unconventional election campaign.

Each candidate heads into the debate facing not only their opponent, but also obstacles of their own making: public vitriol stemming from Trump’s bullying, race-baiting campaign; Clinton’s email trouble, questionable health and an image shaped in part by her husband’s presidency.

A first debate primer:

THE PROGRAM — NBC’s Lester Holt will moderate the 90-minute debate at Hofstra University, which is expected to fetch the biggest audience of a presidential debate ever, possibly matching or exceeding Super Bowl 2015’s 114.4 million viewers. The format will be six 15-minute segments, two of which are dedicated to topics Holt selected: America’s direction, achieving prosperity, and securing America. The rest of the time will be used for rebuttals and deeper discussion of those topics.

RELATED: LIVE STREAM: First Presidential Debate between Clinton and Trump

POLLS — With only 42 days to go until the general election on Nov. 8, anything can make or break these campaigns. The debates are expected to be a significant factor in the general election as the candidates are in a true tie, each with 41 percent among registered voters (and Clinton leading Trump 46 to 44 percent among "likely voters"), according to the Washington Post-ABC News poll released Sunday. The same poll reports that 36 percent of likely voters said there is at least a small chance the debate will change their minds. And three-quarters of the registered voters polled said they will be watching tonight.

PROTESTS — Not all the action will be on the stage. Thousands of protesters are expected to gather in Long Island outside Hofstra; among them is expected to be Green Party candidate Jill Stein, whose numbers did not qualify her for the debate, but whose supporters will stage a “Let Jill Debate” protest and a subsequent “People’s Debate.”  Over 1,500 fast food workers will be there on behalf of a $15 minimum wage. And some from the New York Communists said they will go to rail against Trump.

The other Independent, Libertarian candidate Gary Johnson, will be in New York spreading his message about interplanetary exploration (“The future of the human race is space exploration,” he said Sunday).

ENDORSEMENTS — Heading into the debate each candidate scored a high-profile endorsement. The New York Times' editorial board threw its support behind Clinton. One-time rival for the Republican presidential nomination Ted Cruz gave his endorsement to Trump, even after withholding it during his appearance at the RNC in July.

TACTICS — The big noise yesterday came from reports that the Trump campaign invited Gennifer Flowers, a former-fling of Bill Clinton, to the debate, which they later denied but which succeeded to bring that aspect of Clinton's past into the spotlight. The Clinton campaign defended their invitation to Shark Tank business guru Mark Cuban. The Clinton strategy includes emphasis on fact-checking during the debate, and the general theory that Trump's rash temperament, above all, disqualifies him. 

"When you poke him a little bit, he comes back and attacks whoever is doing it. That's … why he got in so much trouble when he attacked the Gold Star family, the Khan family after right after our convention. That's what he does, that's who he is, that's why he's dangerous and unpredictable," Clinton adviser John Podesta said on NBC.

Screens and Streams

TV - All three presidential debates will air on most prime networks: ABC, NBC, Fox, Fox News, CBS, MSNBC, Univision and C-SPAN.

Online: The digital news providers will also have livestreams: Buzzfeed News, Huffington Post, Politico, Yahoo, Telemundo, The Daily Caller will be broadcasting along with the simulcasts offered online by most of the prime-time broadcasters listed above.

Social – Social media has plenty of alternative ways to appreciate and join the debate: Twitter teams with Bloomberg for a livestream, Snapchat will be pumping out Live Stories. Facebook Live will feature other journalists and audience members at Hofstra.

For the most fun: Clever apps that will make your debate experience even more fun include “HillaryDonald” which allows you to virtually “boo” or “cheer” the candidates by shaking or tapping your phone – those stats are translated into live maps and graphs showing the reaction from around the country. Another one, called TwitterScope combs tweets for negative and positive sentiments and pumps out scores. 

2016 Debate Schedule:

1. Clinton vs. Trump: Monday, Sept. 26 – Hofstra University, Hempstead, NY

2. Kaine vs. Pence: Tuesday Oct. 4 – Longwood University, Farmville, VA

3. Clinton vs. Trump: Sunday Oct. 9 -  Washington University in St. Louis, Saint Louis, MO

4. Clinton vs. Trump: Wednesday, Oct. 19 – University of Nevada, Las Vegas, NV