(AP Photo/Charlie Neibergall)
Former Secretary of State Hillary Clinton walks onto the debate stage.

The most talked about moment on Twitter from the first Democratic debate in Las Vegas, Nevada, last month occurred just under an hour into the affair.

Sen. Bernie Sanders (I-Vermont) had a prime opportunity to attack the front-runner, Hillary Clinton, about her use of her personal email account while serving as secretary of state.

But he not only demurred. He defended.

"Let me say something that may not be great politics. I think that the secretary is right. The American people are sick and tired of hearing about your damn emails!" Sanders said.

The interaction set the stage for what was a fairly non-confrontational debate, as the candidates essentially fashioned the best parts of their stump speeches into responses to CNN moderator Anderson Cooper's questions.

But if the past few weeks are any indication, the second debate won't be nearly as cordial as the first.

On Sunday, Yahoo's Hunter Walker reported that an aide to Sanders threw a "bizarre" fit over CBS' decision to increase the focus of the debate on foreign policy following the deadly attacks in Paris, France that left at least 129 dead.

According to CNN, Sanders aide Mark Longabaugh "completely lost it."

"This wasn't part of the deal. It shouldn't be allowed. It wasn't advertised as a foreign policy debate and it's turning into a debate it wasn't supposed to be," CNN reported Longabaugh as saying.

Since October, Sanders, Clinton, and former Maryland Gov. Martin O'Malley (D) — the three candidates left in the Democratic field — have begun to take not-so-subtle jabs at each other, setting the stage for conflict at the debate.

The New York Times reported that Sanders — who has said repeatedly that he will refrain from attacking his Democratic rivals during the race — will likely go after Clinton more forcefully during the debate this time, if prompted.

For her part, Clinton has already amped up her criticism of Sanders.

Referencing the Vermont senator's plea during the first Democratic debate for Americans to tone down the rhetoric over gun control and "stop the shouting," last month Clinton accused Sanders of gender bias.

"I've been told to — and I quote — 'stop shouting' about gun violence. First of all, I'm not shouting. It's just sometimes when women talk, people think we're shouting," Clinton said.

According to Politico, Sanders believes that Clinton went a step too far with her criticisms. He has been inspired to go on the offensive by highlighting Clinton's flip-flopping on issues like the Keystone XL pipeline, gay marriage, and campaign-finance reform.