With the Democratic presidential primary in its twilight, frustration within the ranks over the party's handling of the primary process spilled out this week as Bernie Sanders supporters lashed out at party leaders, arguing that their candidate has been treated unfairly.

The public outpouring of anger began last weekend at the Nevada Democratic Party convention, where Sanders supporters who said Hillary Clinton's backers had subverted party rules shouted down pro-Clinton speakers and sent threatening messages to state party Chairwoman Roberta Lange after posting her phone number and address on social media.

That led Democratic National Committee Chairwoman Debbie Wasserman Schultz, Senate Minority Leader Harry Reid and other top party leaders to demand an apology and publicly ruminate on the possibility of violence at the Democratic National Convention in July as they prepare for a general election battle with Donald Trump.

Obama administration officials on Wednesday played down concerns about escalating tensions, likening the race to the 2008 primary fight between Clinton and then-Sen. Barack Obama.

But Sanders isn't backing down. A campaign spokesman said Wednesday that the campaign was "looking into" whether to ask for a recount in Kentucky, where Sanders narrowly lost on Tuesday night, and he fired up his crowd in Southern California Tuesday night by calling out the Democratic establishment.

The Sanders campaign on Tuesday did condemn unruly behavior from supporters and those who made threats to party leaders, but made clear it is sticking with its stance that the party is subverting the process in a way that benefits Clinton.

"These claims that our campaign is sort of fomenting violence in some way are absolute nonsense," Sanders campaign manager Jeff Weaver told CNN's Wolf Blitzer Tuesday night, adding that the campaign "absolutely, categorically" condemns any threatening behavior.

The breakdown in civility comes after what has otherwise been a comparatively polite campaign season for Democrats, but the frustration exposes a rift in the party and undercuts the notion that Clinton will be able to march into the Democratic convention this summer with a party unified behind her.

Throughout the year, Sanders and his supporters have complained about the nomination process and ways they believe it has helped Clinton, including debates held on Saturday nights, closed primaries in major states such as New York, and the use of superdelegates -- essentially free-agent party and union stalwarts who are overwhelmingly backing Clinton.

"The problem is that there are long-simmering concerns about unfair treatment out in the Nevada Democratic Party," Weaver added. "We are not going to allow the millions of people who supported Bernie Sanders to be sort of rolled over in places like Nevada by the way they handled that convention."

JUST WATCHED Sanders campaign manager on Nevada convention Replay More Videos ... MUST WATCH Sanders campaign manager on Nevada convention 06:58

Earlier on Tuesday, Sanders released a statement suggesting that his supporters were justified in feeling like the party has given them a raw deal.

"If the Democratic Party is to be successful in November, it is imperative that all state parties treat our campaign supporters with fairness and the respect that they have earned," Sanders' statement read. "Unfortunately, that was not the case at the Nevada convention. At that convention the Democratic leadership used its power to prevent a fair and transparent process from taking place."

In an interview with CNN, Wasserman Schultz said that statement wasn't enough.

"I was deeply disturbed," she said. "The senator's response was anything but acceptable. It certainly did not condemn his supporters for acting violently or engaging in intimidation tactics and instead added more fuel the fire."

Sanders campaign: Wasserman Schultz 'throwing shade'

The DNC chairwoman, however, said she has not spoken directly with Sanders, but that her staff has been in touch with the Vermont senator's campaign. She also pushed back against Sanders' accusation that the party had rigged the system against him.

"We've had the same rules in place that elected Barack Obama. These rules were adopted for state parties all across the country in 2014," she said. "They were followed and even if the Sanders supporters were frustrated, there is never, under any circumstances, a place for violence and intimidation to be resorted to in response."

JUST WATCHED The Kentucky primary on CNN in 60 seconds Replay More Videos ... MUST WATCH The Kentucky primary on CNN in 60 seconds 01:07

On CNN's "New Day" Wednesday morning, Weaver accused the DNC chairwoman of "throwing shade."

"We can have a long conversation about Debbie Wasserman Schultz and how she's been throwing shade on the Sanders campaign," Weaver said.

"I gotta say it's not the DNC," he added. "By and large the DNC has been very good to us, but not Debbie Wasserman Schultz."

Wasserman Schultz brushed off Weaver's comments later in the day.

"My response to that is hashtag SMH (shake my head)," Wasserman Schultz told Blitzer on "Wolf." "We need to focus on one thing: get through this primary and work to prepare for the general election and make sure that we can continue to draw the contrasts between either one of our really fine candidates who are focused on helping people reach the middle class and make sure that we get equal pay for equal work and create jobs and not let the Republicans take health care away from 20 million Americans."

'He should get things under control'

Speaking to reporters in Columbus, Ohio, on Wednesday afternoon, Vice President Joe Biden said if such disruption happens again, "He's going to have to be more aggressive in speaking out about it."

"But here we are in May, as was pointed out," Biden continued. "Hillary was still in this in May, in June. I'm confident that Bernie will be supportive if Hillary wins, which the numbers indicate will happen. So I'm not worried. There's no fundamental split in the Democratic Party."

Leading congressional Democrats also pushed Sanders to rein in his supporters. Reid called Sanders' response "a test of leadership" for Sanders, and a source in his office told CNN that the Nevada senator is waiting to hear from the senator himself on the matter.

"The convention was Saturday. It's now Wednesday afternoon. And he hasn't spoken about it," the source said.

California Democratic Sen. Barbara Boxer, who spoke on behalf of Clinton at the Nevada convention, condemned the behavior.

"He should get things under control," Boxer said. "We're in a race that is very critical. We have to be united."

"This is a character moment for Bernie Sanders. He's got to figure out how he's true to his values and his ideals fully," said CNN political commentator Van Jones.

"I think Hillary and Bernie both misunderstand this movement. I think Hillary just sees it as just a bunch of rowdy kids that at some point will just calm down and fall into line," he said, later adding, "I think Bernie actually only sees the good in his followers. I think Bernie really misunderstands there is a nasty edge to his following that he's not taking seriously enough."

Sen. Tim Kaine criticized Sanders' responses in the wake of reports that Democrats felt threatened at and following the convention.

"What he did yesterday was sort of say it's the party's fault," Kaine told CNN. "That deflection of responsibility is not leadership."

Kaine added that the angry protests could be "dirty tricksters in the crowd" and not just Sanders' supporters.

"I don't think we should assume that all of the people raising hell are Bernie people," Kaine said.

Sanders goes after the establishment in fiery speech

Speaking in Southern California Tuesday night, Sanders fired up the crowd by calling out the Democratic leadership.

"The Democratic Party is going to have to make a very, very, profound and important decision. It can do the right thing and open its doors and welcome into the party people who are prepared to fight for real economic and social change. That is the Democratic Party I want to see." Sanders said.

"I say to the leadership of the Democratic Party: Open the doors, let the people in! Or the other option for the Democratic Party, which I see as a very sad and tragic option is to choose and maintain its status quo structure, remain dependent on big money campaign contributions and be a party with limited participation and limited energy," he said.

The crowd responded by chanting, "Bernie or Bust!" the equivalent of the Republican #NeverTrump slogan for the Democratic race.

His speech barreled through his list of Clinton contrasts, comparing his stances with her (and criticizing those stances) on minimum wage, fracking, breaking up the big banks, and her use of super PACs.

In response to the chaos in Nevada, Clinton campaign manager Robby Mook simultaneously praised the passion and participation of Sanders' supporters while adding that Clinton believes that "no one should be intimidated, harassed or threatened in this process." He called on them to focus that energy on unifying the party, a task that could be difficult given the raw feelings many Sanders supporters have for Clinton after the primary.

"Supporters of both Clinton and Sanders deserve respect for the work they have put into their campaigns," Mook said. Ultimately, we are confident that the passion and energy from the primary will be united in a common purpose -- to move forward the ideals of our party and keep the White House out of Donald Trump's hands."

White House Press Secretary Josh Earnest on Wednesday downplayed any tensions between the two campaigns.

He recalled a similar "tenor" to the 2008 contest between Clinton and Obama, saying those tensions were "no less intense" and didn't lead to a negative result in the general election.

"I think one of the lessons of 2008 is not to confuse passion in primary for disinterest in the general election," he said, adding that "highly motivated" supporters were good for democracy.

Worries about the Democratic Convention

JUST WATCHED Could Sanders revolt upend Democratic convention? Replay More Videos ... MUST WATCH Could Sanders revolt upend Democratic convention? 02:10

While the spotlight this year was on the Republican primary and prospect for a contested convention and protests at the national convention in Cleveland, some Democrats now worry about that happening at their convention in Philadelphia.

Wasserman Schultz said the incidents in Nevada would result in the DNC reviewing its procedures for Philadelphia.

"As a result of this happening this weekend, we will have conversations both at the staff level as well as my having conversations with the candidates so that we can make sure that both campaigns are focused on making sure that we can allow this process for the duration of the primary to play out in a civil and orderly way," she said.

But the DNC chairwoman said she wasn't worried about violence happening at the convention.

"This was absolutely a serious concern, which is why I said what needed to be said yesterday and others have said that there was real concern," Wasserman Schultz told Blitzer Wednesday afternoon. "But it is important and I am confident that the candidates take the messages to heart about making sure that we respond and conduct ourselves in a civil and orderly way."

California Democratic Sen. Dianne Feinstein, however, warned that Sanders' intention to take his candidacy to the Democratic convention could spark unrest similar to the chaotic 1968 convention in Chicago and the riots surrounding it.

"It worries me a great deal," Feinstein told CNN's Manu Raju. "You know, I don't want to go back to the '68 convention, because I worry about what it does to the electorate as a whole -- and he should, too."

And Senate Minority Whip Dick Durbin of Illinois also said he's concerned about violence in Philadelphia.

"We saw what happened at the Trump rallies, which broke into violence, people punching one another. I don't want to see that happen at the Democratic Party," Durbin told CNN. "I call on Bernie to say to his supporters: be fervent, be committed but be sensible. Don't engage in any violence."

Weaver pledged Tuesday night that the convention will be peaceful.

"There's not going to be any violence in Philadelphia, Wolf, I guarantee you that," he said on CNN. "We hope for a fair and orderly convention."