David Sirota, a journalist who's been tapped as Sen. Bernie Sanders' 2020 speechwriter, was working for the campaign in an unofficial capacity for months as he simultaneously attacked other Democratic candidates on Twitter.

According to The Atlantic, Sirota deleted roughly 20,000 tweets on Monday, a day before it was announced he's on Sanders' campaign.

Faiz Shakir, Sanders' 2020 campaign manager, told INSIDER the campaign feels "good about the diversity of thought" among its recent hires.

Sen. Bernie Sanders' new speechwriter for his 2020 campaign has been attacking other 2020 Democratic candidates nonstop for months as an unofficial employee, The Atlantic reported.

David Sirota, a journalist known for his unabashed progressive politics and vicious Twitter-takedowns, has been slamming candidates like Kamala Harris, Cory Booker, Kirsten Gillibrand, and Beto O'Rourke, on social media, his website, and in columns for the Guardian. All the while, he was advising Sanders' campaign in an unofficial capacity while helping him write early campaign speeches, according to The Atlantic report.

Sanders' campaign manager, Faiz Shakir, confirmed to The Atlantic that Sirota was working for the campaign in an undisclosed role in a test trial of sorts to see how the journalist and senator got along. Shakir said this role began on February 20, after Sirota had left a position at the Capital and Main website on February 14, and before he officially signed on with the campaign on March 11, according to CNN.

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The Sanders campaign on Tuesday announced a slew of new hires, including Sirota as a senior communications adviser and speechwriter.

Sirota, who reportedly deleted up to 20,000 tweets on Monday prior to the big announcement, often defended his incendiary attacks on prominent Democrats by citing his position as a journalist.

The Guardian is refuting aspects of The Atlantic's story on Sirota.

Guardian editor John Mulholland on Tuesday tweeted, "Once David was approached by Sanders he wrote nothing else for us" and claimed his "last piece for us" was at the end of December.

The Atlantic story stated, "Since December, David Sirota has, on Twitter, on his own website, and in columns in The Guardian, been trashing most of Sanders’s Democratic opponents—all without disclosing his work with Sanders."

Sirota is not the first Sanders 2020 campaign hire to raise eyebrows on social media.

Belén Sisa, the campaign's deputy national press secretary, recently sparked uproar when in a Facebook discussion she suggested American Jews have "dual loyalty" to Israel. Sisa has since apologized for these comments. Her Facebook page is no longer publicly viewable.

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Meanwhile, old tweets from Sanders' new press secretary Briahna Joy Gray on Russian election interference, a journalist whose role with the campaign was also announced Tuesday, have gained attention on social media. In one such tweet from September 2016, Gray said, "@realDonaldTrump is right. The dems pushed the 'Russia did the hacking' angle b/c it was politically advantageous for them. # debatenight."

This tweet came amid a presidential debate in which then-candidate Donald Trump suggested China or a person who "weights 400 pounds" could've been responsible for hacking the Democratic National Committee. The US intelligence community has concluded the Kremlin, under the direction of Russian President Vladimir Putin, was responsible.

Some reporters have interpreted Gray's tweet as suggesting the hacks were an "inside job," implying there was a conspiratorial element to her statements at the time.

When asked about Gray's tweet, Shakir told INSIDER he "didn't read it that way." In his view, Gray was being critical of the "preponderance" of attention that was granted to Russia at the time and felt it was "distracting from other arguments that could be used to defeat Donald Trump."

"What's past is in the past. We don't own a lot of the views she's articulated in the past," Shakir said.

"Now, going forward, we are all on one team. I have zero doubt she will be a full team player," he added.

When asked for a comment about the general narrative surrounding some of these new hires and some of the immediate criticism, including toward Sirota, Shakir said, "I welcome people digging into Twitter profiles if that’s what they want to do with their time."

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"We're honored to have Briahna on this campaign," Shakir said, adding that the campaign has tried to "put together" a team that "looks like and reflects America."

"We feel good about the diversity of thought in this campaign," he said, contending that Sanders is "doing his part to enlarge his tent" with his campaign hires.

As of Tuesday, 70% of Sanders' national leadership team for his 2020 campaign is comprised of women. The Vermont senator has prioritized diversity in both his rhetoric and campaign staff in 2020 after facing criticism for having a mostly male and white staff in 2016.