Bernie Sanders has brought an old ally back into the fold — one who has a long-running history of painting a number of Sanders’ top 2020 foes as being corporatist traitors.

Sanders’ campaign announced Tuesday that he’s hired journalist David Sirota as a senior communications adviser and speechwriter.

Sirota began his career in populist-liberal politics, working as Sanders’ press secretary back when he was a House member before working for a number of Democratic candidates and elected officials.

But he’s spent the last decade working as a journalist for a variety of liberal publications, where he’s written some scathing takedowns of Sanders’ likely 2020 opponents.

Sirota has been helping Sanders in advisory capacity for months, The Atlantic reported Tuesday afternoon, and hadn’t disclosed his Sanders work while trashing his opponents in recent months. Sirota deleted tweets critical of other candidates after the magazine reached out, and Sanders’ campaign didn’t acknowledge Sirota’s role until asked by the publication.

Sirota has been particularly harsh towards Biden. As Biden geared up for a potential presidential run in 2015, Sirota authored a lengthy piece detailing Biden’s decades-long work to help push through a bankruptcy bill that benefitted Biden’s home-state credit card industry. He’s far from the only reporter to dig into the issue, and it will likely be a problem for Biden on the campaign trail. But that’s not the only criticism he’s leveled at Biden — Sirota has highlighted a number of other issues he takes with the former vice president in various stories and since-deleted tweets in recent months as it’s become clear that Biden would likely be squaring off against Sanders (and before Sirota had publicly acknowledged his role helping Sanders).

Sirota has also been leading the charge against O’Rourke, stoking a growing online feud between Sanders and O’Rourke supporters that has already left its mark on the primary. Sirota went on a recently deleted tweetstorm in December attacking O’Rourke’s supporters in the oil and gas industry, ignoring the fact that O’Rourke’s campaign didn’t accept any corporate PAC money. That came the same week that he penned a pair of columns accusing O’Rourke of voting regularly with President Trump and slamming him for following President Obama’s model of working with big business rather than treating it as the enemy.

He’s also been fiercely critical of Sen. Kamala Harris (D-CA).

“Kamala Harris is entitled to no scrutiny, just like every other Democratic politician who is loved by Clinton donors. Also, war is peace,” he tweeted in 2017 during a since-deleted tweetstorm attacking some of her actions as attorney general.

And he’s taken aim at other more business-friendly Democrats as well, including Sen. Cory Booker (D-NJ).

Those attacks have infuriated establishment Democrats. Center for American Progress head Neera Tanden blasted Sirota at the time of his O’Rourke attacks, for instance, calling them “seriously dangerous” and slamming Sirota for stirring up animosity towards leading Democratic candidates.

Oh look. A supporter of Bernie Sanders attacking a Democrat. This is seriously dangerous. We know Trump is in the White House and attacking Dems is doing Trump’s bidding. I hope Senator Sanders repudiates these attacks in 2019. https://t.co/41A4uxtWGY — Neera Tanden (@neeratanden) December 4, 2018

Sanders allies and others on the left roll their eyes at that criticism, believing that more centrist and business-aligned Democrats just don’t want to face scrutiny of their records.

Democrats of various ideological stripes have been terrified of the prospect of the 2020 primary turning into a repeat of 2016, where deep ideological and personality-driven rifts led Sanders supporters to refuse to back the eventual nominee and helped elect Trump. That’s ironically (and arguably counterproductively) led to some intense sniping between Sanders allies and Hillary Clinton’s former staff, who argue that Sanders’ allies are already sowing divisions that could hurt the eventual nominee, whoever that is. Sanders’ hiring of the fiery Sirota will likely do little to calm those concerns.

Sirota declined to discuss this story on record. Sanders campaign didn’t respond to requests for comment.

This story was last updated at 4:30 p.m. to include The Atlantic’s reporting.