UPDATED Feb. 1, 2016 at 10:40 a.m. ET to reflect a clarified tweet from Emily Nussbaum.

WASHINGTON — Bernie Sanders' rapid response director Mike Casca made a plea to his candidate's supporters this week: Stop with the trolling.

if you support @berniesanders, please follow the senator's lead and be respectful when people disagree with you. — mike casca (@cascamike) January 26, 2016

Sanders has narrowed the gap with Hillary Clinton, in part by harnessing the anger of voters who are unhappy with the status quo.

But with the Iowa caucuses now days away, a subset of Sanders supporters has become extremely vocal. Their messages, which are oftentimes derogatory and misogynistic, are geared at Clinton supporters (or anyone who disagrees with Sanders for that matter). They've even become prominent enough to earn a nickname: the "BernieBros."

"They're a mob, and it's not positive toward their candidate, it's trying to tear you down from supporting Hillary," said one chief of staff for a Democratic member of Congress who's endorsed Clinton.

Now Sanders campaign staffers and supporters are scrambling to counter this BernieBro narrative, saying it's not reflective of what Sanders is about.

"Sen. Sanders is committed to running a positive campaign on the issues, and we encourage all of his supporters to stick to that same message," Hector Sigala, Sanders' digital media director, told Mashable.

The 'BernieBros'

Often young, white and predominantly male, the BernieBros have grown in ranks through r/SandersforPresident, which has become one Reddit's most popular political groups.

Nearly 160,000 online supporters have helped raise money for Sanders' campaign, and even organized phone banks to mobilize voters.

But this has helped breed the hostile BernieBros, who some supporters say could diminish the otherwise groundbreaking organizing that larger group is doing online.

I have enormous respect for Bernie Sanders. Too many of his supporters disrespect him with their behavior on social media — Michael Cohen (@speechboy71) January 26, 2016

"We love our supporters, and we know we wouldn't be here without you all, but it does add a layer of complexity when we have to track what you all do during some moments when we are shaping our messaging," Sigala posted on the subreddit. "Above all: just know you represent our movement and be respectful with those who disagree with you."

That thing where Bernie's rapid response director has to tell Berniebro Twitter to knock it the fuck off. https://t.co/zpIbM5VDFp — Ian Millhiser (@imillhiser) January 26, 2016

"I wish more people on reddit followed this request," user RedditIsAngry replied on the thread. "I feel like the Bernie fans are a hostile group."

'The most obnoxious people' on the Internet

Last week, Sen. Jeanne Shaheen, a New Hampshire Democrat and Clinton supporter, posted this photo on Facebook:

These were the types of comments that followed:

"Their vaginas are making terrible choices!" wrote Scott Lockhart, who later in the thread posted about supporting Sanders.

The so-called "BernieBros" support a progressive candidate who campaigns on gender equality. So why are they attacking women online?

"This is the kind of thing I would expect more in a general election," said Jennifer Lawless, director of the Women & Politics Institute at American University.

"But one of the things that’s a little bit different this election cycle on the Republican side — and also with the Sanders campaign — is that we’re seeing people who are not necessarily that involved in politics getting involved," Lawless said. "So when you broaden the scope of participants, you're bound to pick up bad actors."

Man, the Feel The Bern crew (as opposed to Bern himself) is such a drag. Say anything pro-Hil & they yell "bitch" & "psycho." V idealistic! — emily nussbaum (@emilynussbaum) January 26, 2016

Emily Nussbaum, a tv critic who posted a tweet about the sexist comments from the BernieBros, later clarified her tweet, but stuck to her point that the group is still hyper-aggressive on social media.

it's too late to delete the tweet, so I figured I'd try to explain the context. As is my way, I've done so in a series of confusing tweets. — emily nussbaum (@emilynussbaum) January 30, 2016

Still, the Congress member's chief of staff, said dealing with the "bad actor" Sanders' supporters on her boss' campaign Facebook page has become "exhausting."

"The most obnoxious people are the Bernie supporters on social media," the chief of staff told Mashable.

"We’ve gone out of our way not to be critical of him ever," she said. "But any time she posts anything on her political page about Hillary, we get all of these Bernie people just being ridiculous. And I've talked to staff for other members, and it's just exhausting."

"If I'm a casual supporter, that would've absolutely turned me off. It's just unnecessary."

President Obama even brought up these attacks during an interview with Politico this week.

"The truth is in 2007 and 2008, sometimes my supporters and my staff, I think, got too huffy about what were legitimate questions [Clinton] was raising," Obama said, adding that he sees this manifesting in Sanders' movement in 2016.

As of now, Sanders and Clinton are neck-in-neck in Iowa, and victory will likely come down to who has the better organization.

Will the BernieBros hurt Sanders' electoral chances? It will become clearer on Feb. 1.