The facade of nice is finally starting to fall away from the Democratic presidential primary.

Eleven Republican presidential candidates are expected to rip each other to shreds Wednesday night at their second debate in California. But so far, the Democratic candidates have assumed what sometimes feels like a forced smile as they campaign past each other and largely ignore each other.

But with the candidates hurtling towards their first debate less than a month away and polls showing a closer-than-expected race, the harmony can’t last long.

Long simmering behind-the-scenes tensions between front-runners Hillary Clinton and Bernie Sanders spilled into the public Tuesday, and are likely only to grow hotter as the candidates prepare to confront each other on stage.

Related: Clinton ally slams pro-Warren effort

On Tuesday, the Huffington Post made public the open secret that a pro-Clinton super PAC has been plying reporters with unflattering opposition research about Sanders.

Clinton still refuses to utter the words “Bernie Sanders” in speeches and interviews and her campaign has said nothing negative for the record about the Vermont senator. Instead, Clinton’s team has has largely outsourced the dirty work to the super PAC Correct the Record, which unlike most super PACs coordinates directly with her campaign.

Correct the Record has privately been pushing out negative storylines on Sanders for months, and has deployed so-called trackers to videotape his events, as msnbc previously reported. It’s done the same for Martin O’Malley, who has stepped up his rhetoric against Clinton as he tries to climb out of the single digits in polls.

And on the trail for Clinton, Democratic officials campaigning on Clinton’s behalf have begun pulling out their knives against Sanders. Clinton’s campaign has denied ordering the attacks and they do not appear coordinated in any way, but the barbs have aggravated and emboldened Sanders to be more aggressive than he might have been otherwise.

While he often reiterates his pledge to run a positive campaign, Sanders seems increasingly willing to tangle with the Democratic front-runner.

When Sanders began his campaign, he was more likely to take a shot at a reporter for asking him about Clinton than to speak ill of the candidate herself. Now he appears eager to draw contrasts with Clinton and seems to delight in pointing out her ideological deficiencies.

Appearing Monday at University of Virginia’s Miller Center for a townhall meeting, the senator laid out in a blistering minute-and-a-half some key policy differences between Clinton and himself. After ticking through minimum wage, trade policy, climate change, and Social Security expansion, Sanders summed it up: “Hillary Clinton and I have strong disagreements.”

Later, when asked about whether his lack of foreign policy experience would hinder him as president, he steered his answer toward the Achilles Heel of Clinton’s 2008 campaign.

“To say that I don’t have experience, well, I’m not the former secretary state. But I think my judgment has been pretty good,” he said. “When looking at the information available to Senator Clinton and looking at the information available to Congressman Sanders, I voted against the war in Iraq.”

On Tuesday, he was was quick to respond to news of Correct the Record’s actions, sending not one but two fundraising emails of the incident.

“Yesterday, one of Hillary Clinton’s most prominent Super PACs attacked our campaign pretty viciously,” Sanders wrote in an email to supporters. “It was the kind of onslaught I expected to see from the Koch Brothers or Sheldon Adelson.”

Later, his campaign manager Jeff Weaver followed up. “The billionaire class is terrified,” he wrote. “Now that they’re attacking our campaign directly, we have a choice: we can either stand by and accept business as usual, or we can strike back and send a message that we’ve had ENOUGH of billionaires and corporations buying our elections.”

David Brock, who founded Correct the Record, defended his group’s work in an interview Tuesday with Bloomberg TV. “You’re going to have to draw some contrasts” in a campaign, he said.

And Brock ruled out an apology to Sanders. “Gosh no, this is just standard opposition research. You’ve seen it before and you’ll see it again,” he said.

Related: Clinton’s lead cut in half since August: poll

Even though some donors have urged her to do so, Clinton herself has yet to engage Sanders. That will likely have to wait until the debate. “When we start the debate, we will start to draw contrasts not only as I do all the time with Republicans but where appropriate with my Democratic competitors,” she told reporters while campaigning Monday in Iowa.

Still, she has recently begun adding lines to her comments that could be read as subtle digs at Sanders. “You can wave your arms and give a speech, but at the end of the day are you connecting with and really hearing what people are either saying to you or wishing that you would say to them?” she told NBC’s Andrea Mitchell.

When Sanders and Clinton face off in mid-October, however, there will be no way to avoid real combat.