It became clear within the first 10 minutes of the Democratic debate on Sunday night that Bernie Sanders had had just about enough with Hillary Clinton’s attacks.

And if the two Democratic frontrunners liked each other before this race, those feelings were long gone.

It began, appropriately enough, with a gun fight. As the first votes of the 2016 Democratic primary are only weeks away, Clinton has pounded Sanders relentlessly for his past position on guns—insinuating in a recent ad that he was siding with the gun lobby over President Obama’s push for more gun regulations. (She never mentions Sanders’s name, of course, but it’s one wink short of “rhymes with Manders.”)

Asked about Clinton’s attempts to tie him to the gun lobby, Sanders came out swinging.

“Well, I think Secretary Clinton knows that what she says is very disingenuous,” he said. “I have a D-minus voting record from the NRA.”

Clinton hit back, invoking the murders of nine African Americans last year in their Charleston church by a white man with federal charges pending against him, which should have prevented him from getting a gun.

“He voted for what we call the Charleston Loophole,” she began, sounding just a tad rehearsed. “He voted for immunity from gunmakers and sellers, which the NRA said was the most important piece of gun legislation in 20 years.”

Clinton accused Sanders of turning a blind eye to gun regulations until this race. “Now, I am pleased to hear that Senator Sanders has reversed his position on immunity and I look forward to him joining with those members of Congress who have already introduced legislation,” she said.

As she spoke, Sanders unsuccessfully tried to mask his grimaces with smiles, and seemed to be just waiting for the opportunity to hit back. And he did when asked why the former secretary of state was out-polling him with minorities two to one.

“Well, let me talk about polling,” Sanders began, channeling his inner Donald Trump. “As Secretary Clinton well knows, when this campaign began she was 50 points ahead of me. We were all of three percentage points. Guess what? In Iowa, New Hampshire, the race is very, very close. Maybe we’re ahead New Hampshire.”

And it’s true. It’s a scenario made of night terrors for Clinton, who, eight years ago, lost Iowa to another insurgent progressive who her campaign drastically underestimated. (If you listen closely you can almost hear her whispering softly “one, two, three, wake up.”)

That’s not to say she’ll lose the nomination if Iowa (and New Hampshire) go for Sanders—South Carolina (Clinton is ahead by 40 points in the Real Clear Politics average), Nevada, and the SEC primary states should provide a sturdy firewall, particularly as the electorate becomes more diverse.

But it could slow her roll into the nomination, and as much as her campaign says they had always expected a tough race, it’s tough to believe anyone ever considered Sanders, who resembles Doc from Back to the Future but is more into social experiments than actual experiments, as a real challenge.

Former Maryland governor Martin O’Malley was also present at the debate, and spent much of it begging for air time when he wasn’t taking swipes at Clinton.

Sanders, for his part, has played the same game as Clinton in recent days—releasing an ad last week in which he said, “There are two Democratic visions for regulating Wall Street, one says it’s OK to take millions from big banks and then tell them what to do.”

The ad, clearly directed at Clinton but playing it cute like her gun ad, was immediately condemned on a conference call with Clinton staffers who were shocked—nay, aghast—that Sanders would go negative. They pointed to Sanders’s pledge to remain positive—and accused him of becoming more of a politician by the day.

And Sanders took the opportunity to knock her again when asked about the ad on Sunday night.

“Well, the first difference is I don’t take money from big banks. I don’t get personal speaking fees from Goldman Sachs,” Sanders said, a reference to Clinton’s pricey speaking fees during her time out of office.

He then went on to describe his belief that the large financial institutions should be broken up.

Clinton countered that Sanders was so far left he even opposed Obama, who helped push through Wall Street reforms like Dodd-Frank.

She can take the attacks, Clinton began, but when Sanders started to attack Obama—that was too far.

“Senator Sanders called him ‘weak,’ ‘disappointing.’ He even, in 2011, publicly sought someone to run in a primary against President Obama,” she said. “Now, I personally believe that President Obama’s work to push through the Dodd-Frank… I’m going to defend Dodd-Frank and I’m going to defend President Obama for taking on Wall Street, taking on the financial industry and getting results.”

It wasn’t the first time during the debate that Clinton hugged President Obama tight and spoke in fawning tones that are sure to be a part of any general election campaign against her. Most notably she dodged a question about whether Obama should have made good on his “red line” with Syria. In the past she has made it clear that their approaches to Syria where different, but not at this debate.

Still, through the back and forth between him and Clinton, there was one place Sanders refused to go: President Bill Clinton’s infidelities.

When asked by NBC’s Andrea Mitchell whether he regretted calling President Clinton’s “past transgressions, quote, ‘totally, totally, totally disgraceful and unacceptable’” Sanders dismissed it in the same fashion he dismissed Clinton’s “damn emails” months ago.

“That question annoys me,” he said. “We’ve been through this. Yes, his behavior was deplorable. Have I ever once said a word about that issue? No, I have not. I’m going to debate Secretary Clinton, Governor O’Malley, on the issues facing the American people, not Bill Clinton’s personal behavior.”