Hillary Clinton and Bernie Sanders. AP For months, when Sen. Bernie Sanders (I-Vermont) had a clear shot at Hillary Clinton, he pretty much stepped aside.

Take the first Democratic debate. Sanders was asked about Clinton's use of a private email sever while secretary of state, an issue that's dogged her campaign and raised issues about her trustworthiness.

"The American people are sick and tired of hearing about your damn emails!" Sanders exclaimed, as the audience cheered.

"Me too, me too," Clinton said, laughing.

Clinton, for her part, had all but avoided even saying Sanders' name on the campaign trail, hoping to avoid riling up a support base that she'll need in a general election.

But with the race beginning to tighten, both candidates have stopped playing nice. Sanders is surging in polls of Iowa and firmly ahead in New Hampshire, the first two nominating states. He has also sharply closed the gap nationally.

It's unlikely that their next meeting, at Sunday's Democratic debate in Charleston, South Carolina, will be cordial. Already, Democrats are bracing for a nastier debate.

"This is no longer a hypothetical," a top Democratic strategist unaligned with a presidential campaign told Business Insider. "I think that Clinton is going to be a lot more aggressive. I think she’s going to make the contrast."

"It’ll be interesting to see — Sanders has never shrunk from a fight. I mean he’s pretty abrasive to begin with, even when he’s trying to be nice. So I think those two together will make for more fireworks."

People close to the Sanders campaign have said recently that given the nasty rhetorical turn in both camps, it's hard to see how the debate itself won't be nastier than past meetings. Some Clinton campaign officials also told Business Insider on background that they're prepared for the possibility.

That's with good reason: In the past two weeks, Sanders and Clinton have taken a slew of direct shots at each other.

Clinton has raised questions about Sanders' tax plan, drawn sharp contrasts with him on guns, and called on him to release a full accounting of how he'll pay for a universal-healthcare plan, which she says would "end all the kinds of healthcare we know."

"And so I’m just saying that we’re only engaging in substantive differences. And I think that’s what you’re supposed to do when you’re in a contest as important as this one. What’s your policy? What’s my policy? How do you defend it? How do you explain it? And then we let the voters make up their minds," Clinton told MSNBC's Rachel Maddow in an interview recently.

Sanders has been quick to respond to the attacks. He has said Clinton's campaign is "in trouble." And his campaign has even taken to Twitter on multiple occasions to troll Clinton and some of her top staffers over their previous support for universal healthcare.

The shift in tone has been notable, in part because of how relatively mum the Democratic side of the race has been up until this point.

Earlier this year, Sanders said that he wouldn't go negative on Clinton out of principle, and out of a political calculation that he had more to gain by staying above the fray.

“I know the media would like me to attack Hillary Clinton and say all kinds of terrible things and tell the world that I’m the greatest candidate in the history of the world and everybody else running against me is a jerk and terrible, awful people," Sanders said. "Nobody believes that stuff.”

Sen. Bernie Sanders (I-Vermont) in his ad. YouTube/Sanders campaign/screenshot

But during the past week alone, the Clinton and Sanders campaigns both released ads that appeared to implicitly criticize the other on issues of gun control and Wall Street regulation.

Clinton appeared on camera in her ad, released ahead of President Barack Obama's State of the Union address. She declared that she was "with" Obama on the issue of holding gun manufacturers liable when crimes are committed with their products, which has been a source of campaign controversy for Sanders.

Sanders also went direct to camera in a spot that took an indirect shot at Clinton's support of Wall Street. It led the Clinton campaign to hold a conference call with reporters and accuse Sanders of breaking a pledge not to run negative ads during the campaign.

"We were very surprised today to see that Bernie Sanders had launched a negative television advertisement against Hillary," Clinton campaign manager Robby Mook said on the call.

"We were particularly surprised because he had personally pledged — and his campaign had pledged — never to run a negative advertisement," he added.

In a statement to Business Insider responding to that Clinton campaign call, Sanders spokesman Michael Briggs suggested that his team rejected the idea that Sanders had directly targeted Clinton.

"This is not an ad directed at Secretary Clinton exclusively. It's about people in the Democratic establishment who believe you can take Wall Street's money and then somehow turn around and rein in the greed, recklessness, and illegal behavior," Briggs said.

Briggs went on to sharply contrast the Clinton and Sanders approaches toward Wall Street regulation.

"Obviously she is part of the establishment that Wall Street has showered with financial support. Bernie is not," he continued. "She wants Wall Street and corporate special interests to like her. Bernie does not. Bernie is showing that you can take on the corrupt campaign-finance system."

Democratic insiders say that Sanders rise has clearly struck a nerve within the campaign.

"The race is tightening," a top Democratic strategist told Business Insider. "And the reason that you know that it is tightening is that in the last week, I’ve seen more attacks from Clinton and pro-Clinton group on Bernie Sanders than in the months prior combined. There’s been a lot more communication among Clinton supporters, more talking points — they’ve just been more aggressive."

Colin Campbell contributed reporting.