It's a departure from the two candidates' unspoken peace treaty; up until this point in the campaign, they have avoided directly challenging each other, preferring to talk around their differences instead. But Clinton on Tuesday served notice that she's willing to use precious time on stage slamming a dark-horse challenger who's giving her a run for her money.
Here's a running list of Hillary Clinton's jabs at Sanders so far:
"I'm a progressive"
One of the debate's opening questions had host Anderson Cooper asking Clinton to state whether she is a progressive or a moderate.
"I'm a progressive," she replied. "But I'm a progressive who likes to get things done."
The well-executed line was meant to be a comparison between her more moderate policies for issues like Wall Street reform and debt-free college and Sanders's much more unusual approach -- he essentially would take government control of many industries like public colleges and health care.
"We are not in Denmark"
One of Sanders's goals Tuesday night is to explain to Americans what, exactly, a socialist is and what a socialist wants to do for America.
He launched into that pitch right away, saying a socialist is someone who wants to spread the wealth from among the top 1 percent to the rest of the 99 percent to create a more equal society. He resisted calling himself a "capitalist" for the second time this week.
Sanders also drew a comparison to the Nordic country of Denmark, which in exchange for higher taxes provides a wealth of social services for its residents.
Clinton had a pithy response: "We are not in Denmark."
Again, it was an attempt to tell voters she thinks her prescriptions for America are much more realistic than Sanders's. She also embraced the ideal of capitalism with regulations to keep it from getting out of hand. It was an early line in the sand.
"It wasn't that complicated to me"
Sanders differs from the left on one major issue: He doesn't have a strong progressive gun-control track record. That's in part because he's represented a rural, gun-owning state in Congress for more than two decades.
Cooper set this back-and-forth up directly when he asked Clinton if she thought Sanders was tough enough on guns.
"No, not at all," she replied, going on to get her biggest applause line of the night out of it: "I think we have to look at the fact that we lose 90 people a day from gun violence. This has gone on too long, and it's time the entire country stood up against the [National Rifle Association]."
She wasn't done with Sanders, though. Clinton had studied his record and was ready with the fact that Sanders voted five times against the Brady Bill, a 1993 law her husband, President Bill Clinton, signed requiring federal background checks for purchases from federally licensed gun dealers.
She still wasn't done. Sanders voted for a 2005 law giving gun manufacturers and dealers immunity from being sued in the wake of gun accidents. Clinton directly linked Sanders's vote for that immunity bill to letting the hated NRA off the hook:
"I voted against it. It wasn't that complicated to me," she said. "It was pretty straightforward to me that he was going to give immunity to the only industry in America. Everyone else has to be accountable, but not the gun industry."
"My plan is more comprehensive"
During a discussion about Wall Street reform, Cooper pointed out that Sanders has called for breaking up big banks. Clinton, by contrast, says that's not the answer: We should charge banks more and keep the Dodd-Frank regulation system in place, she's argued.
Clinton has come under scrutiny from her time as a senator in New York for being too cozy to Wall Street, so she had a lot to prove in her answer to Cooper. Which is one reason why she said:
"Well, my plan is more comprehensive [than Sanders's]. And frankly, it's tougher."
Sanders literally scoffed at the podium next to her. But Clinton went on: "There's this whole area called 'shadow banking.' That's where the experts tell me the next potential problem could come from."
When it was Sanders's turn to speak, he said, "[The] largest banks in America -- are much bigger than they were when we bailed them out for being too big to fail, we have got to break them up."
Clinton responded that she respected his "passion and intensity," but went on to detail why she's the one to take on the big banks as president.
Sanders got the last word in this debate when he said: "Secretary Clinton, you do not -- Congress does not regulate Wall Street. Wall Street regulates Congress."