A prominent liberal commentator and journalist hired as an adviser to Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE’s (I-Vt.) nascent presidential campaign spent months attacking the candidate’s Democratic opponents, even as he was advising Sanders in an informal capacity.

Sanders’s campaign announced Tuesday that it had hired David Sirota, a well-known liberal commentator and radio host, as a senior communications adviser and speechwriter.

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But a

report published by The Atlantic

on Tuesday showed that Sirota had been working for Sanders in an unofficial capacity for months.

During that time, Sirota sharply criticized other Democrats, including former Rep. Beto O’Rourke (D-Texas), who just announced a White House bid last week.

It's unclear if Sirota was being paid for his work prior to his formal hiring to the campaign.

A spokesperson for Sanders's campaign did not immediately respond to The Hill's request for comment.

After he was contacted by The Atlantic on Monday about his work for Sanders, Sirota reportedly deleted more than 20,000 posts from his Twitter account.

Sirota’s informal work for Sanders was previously unknown. Sanders’s campaign only announced Sirota’s formal hiring on Tuesday.

Faiz Shakir, the senator’s campaign manager, confirmed to The Atlantic Tuesday afternoon that Sirota had been working in an advisory role on the campaign for several months.

Sirota often waved off criticism of his attacks on other Democrats by saying that he was simply evaluating candidates as a journalist.

In tweets, Sirota dismissed critics as “deranged” or “mentally incapacitated,” according to excerpts of the posts published by The Atlantic.

Sirota directed his ire at O’Rourke in particular, going after the El Paso Democrat’s voting record in Congress.

But he also criticized other Democratic presidential hopefuls, including Sens. Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (Calif.), Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (N.J.) Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (N.Y.) and former Vice President Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE.

Sirota’s criticism of candidates while unofficially advising Sanders could prompt tensions in the emerging Democratic primary field, which has largely avoided personal attacks and intra-party fighting so far.

In an email to supporters on Tuesday, Sirota announced that he had joined Sanders’s campaign, saying that the offer was one he could not reject.

“[W]hen Bernie asked me to join his campaign, I felt I could not turn that request down, because I genuinely believe that the future of our country, our planet and our children are at stake in this election,” Sirota wrote.