2. Sanders finally brawls with Clinton...
The Vermont senator in the first debate appeared unprepared or unwilling to go after Clinton with any real vigor.
But Saturday night, Sanders suggested Clinton is in the pocket of Wall Street and tied her vote to authorize the war in Iraq to the rise of ISIS. Clinton, meanwhile, knocked Sanders on his mixed record on guns and suggested his policy proposals are too simplistic.
In the most forceful exchange of the night, Sanders pointed to the millions of dollars in donations Wall Street has funneled to Clinton's campaigns over her political career.
"Why over her political career has Wall Street been a major, the major campaign contributor to Hillary Clinton? Maybe they're dumb and they don't know what they're going to get, but I don't think so," Sanders said. "I have never heard a candidate, never, who has received huge amounts of money from oil, from coal, from Wall Street from the military industrial complex, not one candidate -- 'Oh these, these campaign contributions will not influence me.' But why do they make millions of dollars of campaign contributions? They expect to get something, everybody knows that."
Clinton responded by accusing Sanders of impugning her "integrity," before claiming that Wall Street has supported her
because of the work she did to rebuild the Financial District after it was hit in the 9/11 attack on the World Trade Center.
As she did in the first debate, Clinton hit Sanders on guns, calling the Vermont senator's vote to shield gun makers from some lawsuits a "terrible mistake" and urging Sanders to admit as much, as she has done regarding her Iraq vote.
Sanders responded by suggesting there's little daylight between himself and Clinton and O'Malley on guns.
3. ...but he still holds back
Sanders didn't take the gloves off completely during Saturday night's debate in Iowa.
Instead, he took a pass on one of the biggest points of contention of the 2016 cycle: Clinton's record as secretary of state.
Given a chance to slam Clinton's tenure, Sanders demurred, instead pivoting to his "disagreement" with Clinton over her vote in favor of the Iraq War in 2002, more than six years before she became secretary of state.
Republicans are already picking apart Clinton's track record as secretary of state, but Sanders' simply wouldn't take the bait.
It was reminiscent of Sanders' "damn emails"
moment in the first Democratic debate, when he said Americans are tired of hearing about the controversy over Clintons' use of personal email during her time as secretary of state.
4. The moments that could come back to haunt Clinton
However, Clinton's response to Sanders -- when she beat back the suggestion that she is influenced by Wall Street donors -- could really come back to bite her.
Clinton explained her connection to Wall Street by tying her relationship to her role in helping to rebuild New York's financial district after the 9/11 attacks.
Twitter was ablaze Saturday night over the response, and the Republican National Committee was quick to blast out a best-of compilation of political observers' reactions.
It's not the only moment that could come back to haunt Clinton -- especially if she faces a younger GOP nominee, like Sen. Marco Rubio
Clinton at one point said she is "from the '60s" -- a moment in the debate that could be replayed in political ads down the line suggesting she is part of the past, not the future, of American leadership.
5. Paris sets the tone
The debate Saturday night began with the candidates pausing on stage, taking a moment of silence to remember the victims of the terrorist attacks that ripped Paris apart just a day earlier.
Clinton said while "our prayers are with the people of France tonight...that is not enough" and called for better coordinated efforts to root out ISIS, the radical group that claimed responsibility.
She said Americans should see the terrorist attack as a reminder that the election "is not only about electing a president, it's also about choosing our next commander in chief."
Sanders made brief note of the terrorist attacks at the top of his opening statement, but quickly pivoted to the core message of the campaign: "a rigged economy" that benefits billionaires.
Still, Sanders made a bold claim in his reference to Paris: "Together, leading the world, this country will rid our planet of this barbarist (sic) organization called ISIS."
And O'Malley, after saying his heart goes out "to the people of France in this moment of loss," quickly called for "fresh thinking" and "new approaches" -- a definitive nod to his youth and the generational contrast the 52-year-old offers to Clinton's 68 years and Sanders' 74.
All three candidates refused to say the fight was with radical Islam, instead focusing on using the word "jihadist" to describe the threats spotlighted in the Paris attacks.
6. O'Malley gets a zinger, then shrinks away
O'Malley delivered several strong one-liners throughout the night, notably when he slammed Republican front-runner Donald Trump
as a "carnival barker" for his immigration policies.
But O'Malley also missed a couple of key moments where he could have shined.
His central contention is that it's time for a new generation of leadership -- meaning no more baby boomers like Clinton. But he lacked a strong, convincing response to one of the debate's final questions: When have you been most tested in your life, and how did it prepare you for the presidency?
O'Malley's response conceded that he is untested -- that nothing he's been through as mayor of Baltimore or governor of Maryland comes close to the crises he would face as President.
Trump later responded to O'Malley's insult on Twitter by calling him a "clown."