Personal frustration peeked through as Clinton unloaded new attacks on Sanders over his opposition to the auto bailout and Sanders portrayed Clinton as a candidate straight out of Wall Street central casting.
Sunday was also '90s night, as the candidates essentially re-litigated major political battles of the era -- including NAFTA, the assault weapons ban and crime bill -- through modern eyes.
Here are five takeaways from Sunday's debate:
Sanders waved, shouted, eye-rolled, baited and goaded his way through the debate.
Clinton laid into his opposition to the auto industry bailout. That measure was part of a broader rescue of the financial industry, a point Sanders was only too happy to make by saying: "If you are talking about the Wall Street bailout, where some of your friends destroyed the economy —"
Clinton interrupted: "You know..."
Sanders shot back: "Excuse me, I'm talking."
And then Clinton said: "If you're going to talk, tell the whole story."
Sanders railed against the bailout, saying that he decided to "let the billionaires themselves bail out Wall Street -- it shouldn't be the middle class" -- when Clinton tried to interject again.
"Could I finish? You'll have your turn," he said.
He showed his frustration with Clinton again later, saying: "Can I finish, please? All right?"
The exchange demonstrated a new level of comfort with the hand-to-hand combat of presidential campaigns. But it was also a risky move, making him sound potentially patronizing or dismissive of a candidate who could become the first female president.
This from a candidate who entered the race bragging about never running a negative ad. Sanders might keep their disputes focused on differences of policy -- but at times Sunday night, it looked and sounded personal.
Clinton's nuance vs. Sanders' 'no'
When Clinton was asked about fracking, she launched into a nuanced answer that gave credence to localities, state governments, and more. Her bottom line: There wouldn't be many places where it would be OK under her.
Sanders had a much simpler answer. "No," he said. He doesn't support it. And he said he doesn't care about all the Democratic governors who support it.
This, in a nutshell, is the difference between them. She has nuanced positions that look at the breadth of opinion across the country. She's also keenly aware of the limitations of government, and strains to keep her positions within those limits -- part of what she calls her "responsibility gene."
Sanders has definitive positions that take a look at his ideology. That's the Democrat's choice: nuance or no nuance.
This same difference showed up when the two delved into the Flint water crisis at the debate's outset.
Clinton's big move on stage -- her news-making comment at the debate's outset -- was one she'd resisted for months, arguing it was simplistic. But she went for it Sunday night, saying she agrees with Sanders in saying Michigan Gov. Rick Snyder needs to go. "I agree the governor should resign or be recalled," she said.
But Sanders got bigger applause when he proposed putting an end to the city's water bills
, and refunding what residents there have paid in recent years.
"First thing you do is say, people are not paying a water bill for poison water. And that is retroactive," he said.
Clinton: Sanders wanted to kill Detroit
Clinton saw Sanders' punches over trade deals coming from miles away. That's particularly true for the North American Free Trade Agreement, implemented under Bill Clinton
So she readied an attack that Sanders didn't seem prepared for, going at the Vermont senator for opposing the auto bailout.
"The money was there and had to be released in order to save the auto industry and 4 million jobs and to begin the restructuring," Clinton said. "I voted to save the auto industry. He voted against the money that ended up saving the auto industry. I think that is a pretty big difference."
Then, she ticked off a list of states where she thinks that vote will hurt Sanders.
"Given the terrible pressures that the auto industry was under and that the middle class of this state and Ohio and Indiana and Illinois and Wisconsin and Missouri and other places in the Midwest were facing, I think it was the right decision to heed what President-elect Obama asked us to do," she said. "You were either for saving the auto industry or against it. I voted to save the auto industry and I'm very glad that I did."
The damage was done: When Clinton dropped the auto bailout bomb, the audience audibly ooooohed, highlighting the potency of that argument in the home of the U.S. auto industry.
Sanders' blind spots
Sanders clearly learned lessons from earlier debates. He was careful not to suggest President Barack Obama
hasn't done enough -- on anything -- and instead said the country needs to build on Obama's progress.
His criticism that Clinton is pandering to black voters by cozying up to Obama was gone entirely. Sanders was commanding in discussing the Flint water crisis, taking Clinton's push for a debate in the city and turning it into his strong suit. He also took back ground on gun violence, saying of the shootings at Sandy Hook Elementary School in Connecticut
, "You're not the only person whose heart was broken."
But he had his tone-deaf moments, sparking outrage on social media when he seemed to suggest that black people grow up poor and in ghettos and white people do not -- a particular eye-raiser because he'd been asked about his racial blind spots.
"When you're white, you don't know what it's like to be living in a ghetto, you don't know what it's like to be poor," Sanders said.
For Sanders, this is the central challenge facing his campaign. Clinton blew him out across the South among African-Americans, and Sanders can't withstand her doing so again in big, Midwestern states.
Michigan's March 8 primary will be a key test of whether Sanders can win with a more diverse electorate. The following week -- when Ohio, Illinois, Missouri, Florida and North Carolina vote -- will be his moment of truth.
Near the end of the debate, Sanders also cracked a joke about boosting funding for mental health, saying that "when you watch these Republican debates, you know why we need to invest in mental health."
It was the right target (Republicans) and the right audience (liberals) -- but perhaps the wrong topic, as he again risked appearing insensitive.
One for the base
The debate was a strong sign that both candidates still see room to gain or lose ground among liberal voters. They spent so much time jockeying to get to each other's left that there was virtually no talk of Republicans at all.
Clinton and Sanders defended government spending and intervention, teachers' unions, gun control, clean energy programs and efforts to fight climate change. They talked about a beefed-up role for the Environmental Protection Agency.
There was no talk about foreign policy, the deficit, entitlements -- subjects always front-and-center at Republican debates.
This is why Vice President Joe Biden
didn't run for president: winning over the Democratic electorate is a sprint to the left. It's why Trump's strength among white, working-class men causes some Democrats to fret.
It's a clear sign that the Sanders camp doesn't see the Democratic nominating contest ending anytime soon, with liberal bastions like New York (which votes in April) and California (June) available as opportunities to rack up lots of delegates.
Increasingly, Clinton is eyeing the general election on the campaign trail -- axing her usual shots at Sanders from her stump speech, focusing on the economy and laying into Republican front-runner Donald Trump. That wasn't apparent Sunday night.