Here's five moments that mattered:
1. YOUNG SUPPORTER LEANING TOWARDS SANDERS CHALLENGES CLINTON'S HONESTY:
Repeatedly over the years, Clinton has faced low polling on honesty, favorability and trustworthiness -- and tonight a young voter, who said he was leaning towards Sanders, confronted her about it.
“I've heard from quite a few people my age that they think you're dishonest, but I'd like to hear from you and why you feel the enthusiasm isn't there,” Taylor Gipple asked Clinton, referring to the huge support Sanders has among young people.
Despite the uncomfortable exchange, Clinton launched into a lengthy, animated response, highlighting her record as a progressive. "I've been around a long time. People have thrown all kinds of things at me. And you know, I can't keep up with it. I just keep going forward,” she said, later adding with moxie, "You have to have somebody who is a proven, proven fighter. Somebody who has taken them on and won. And kept going and will do that as president. That's why I hope you'll reconsider.”
2. DEMOCRATIC SOCIALIST CANDIDATE CONFIRMS (AGAIN) "YES WE WILL RAISE TAXES"
“There's a little bit of disingenuity out there,” Sanders continued. “We may raise taxes, but we are also going to eliminate private health insurance premiums for individuals and for businesses.”
Sanders and his campaign estimate that his plan would cost $1.38 trillion a year, and that a a family of four making $50,000 a year would save over $5,000 a year, paying less than $500 in taxes for the program, but then theoretically, no longer paying for private insurance, copays and deductibles. He would also raise income taxes for the top earners to pay for the plan.
3. BERNIE SANDERS: THE FORMER TRACK STAR
The 74-year-old senator, who would be the oldest president ever elected, took a moment to challenge the CNN moderator to try to keep up with him.
“Have a seat, senator. I'm tired following you around there,” CNN’s Chris Cuomo said after Sanders stood for a lengthy answer.
“Follow me around today, you'd be a lot more tired,” Sanders retorted (he did have four campaign events in four Iowa cities before the forum). Asked specifically about his past athletic accomplishments, Sanders was not shy.
“I was a very good athlete. I wouldn't say great athlete. Pretty good basketball player. My elementary school in Brooklyn won the borough championship,” he said, though then he seemed to fane a little modesty and add that it was hardly worth mentioning. “I did take third place in the New York City indoor one mile race...I was a very good long-distance runner. Not a great runner, but captain of my track team,” he went on making his case that he has been -- and is still -- in good health.
4. BERNIE LITERALLY STANDS UP TO HILLARY CLINTON
During the forum, CNN played a campaign ad from Clinton camp that painted her as better prepared than her opponents to be commander in chief. Sanders stood up from his chair, said Clinton was a very good person, but "this calls for a standing up response." He went on to list policy differences between himself and Clinton on Wall Street, healthcare, taxes and guns.
On guns, Sanders said "today, Hillary Clinton is running a lot of advertisements on gun issues. Interestingly enough, she's running most of them in New Hampshire, where she thinks it will work. Not running so many of them in rural Iowa."
If polls are any indicator, it is highly likely that in several Iowa precincts there will be too few Martin O’Malley supporters in the room to earn a single delegate, so those people will most likely realign with either Clinton or Sanders. But when asked about this critical part of the complicated and fluid Iowa Caucus, the former Maryland governor did not take the bait, instead standing confidently behind his candidacy.
“My message to the O'Malley supporters across this state is this: Hold strong at your caucus. Hold strong at your caucus because America's looking for a new leader. America's scanning the horizon. We cannot be this fed up with our gridlock, dysfunctional national politics and think that a resort to old ideologies or old names is going to move us forward,” he said. “I tell my people, hold strong. I know this is a tough fight. But I've always been drawn to a tough fight.”