On Thursday, Jack Dorsey made it official: He’s taking on Mark Zuckerberg.

Dorsey, the CEO of Twitter, announced that his company would no longer accept political ads starting in late November. As part of the announcement, he offered a barely veiled shot at Zuckerberg, the CEO of Facebook.

“For instance, it‘s not credible for us to say: “We’re working hard to stop people from gaming our systems to spread misleading info, buuut if someone pays us to target and force people to see their political ad…well…they can say whatever they want! ” Dorsey tweeted.

The announcement backs up comments from Dorsey at a Twitter event last week that criticized a recent address by Zuckerberg at Georgetown University, in which Zuckerberg laid out the free-speech principles that supported his decision to allow campaign ads to include misleading or inaccurate information.

The clash between two of tech’s best-known leaders underscores a growing split in how tech companies are balancing their roots as open platforms with a growing acceptance that their algorithm-powered feeds and personal data-targeted ads have downsides that require companies to take serious steps to stop misuse.

And while Dorsey vs. Zuckerberg is a David vs. Goliath story, considering the respective size of their companies, some in the tech world believe that Facebook’s stance is unsustainable.

“This is a clash of egos,” said Dipayan Ghosh, a fellow at the Harvard University Shorenstein Center on Media, Politics and Public Policy. (Ghosh formerly worked on privacy and public policy issues at Facebook.) “It is Jack vs. Mark — and Dorsey has already taken the high road by placing the democratic interest over profit.”

Both Twitter and Facebook have faced criticism for years for not doing enough to regulate how their platforms are used, and have changed their policies to crack down on hate speech, bullying, abuse and government-led misinformation campaigns.

But Facebook drew a line over how much it was willing to limit the political use of its platform, leading to some of the loudest criticism in the company’s history — a high bar considering the company has pinballed from one scandal to another over the past 20 months.

The decision not to remove false or misleading political campaign advertising has drawn rebukes from politicians, civil rights organizations, consumer advocates and even Aaron Sorkin, the screenwriter of “The Social Network,” a movie about Facebook’s origins.

“You and I want speech protections to make sure no one gets imprisoned or killed for saying or writing something unpopular, not to ensure that lies have unfettered access to the American electorate,” Sorkin wrote in an op-ed article published Thursday in The New York Times.

Dorsey also has some unlikely allies — a vocal contingent of Facebook employees. That dissent from inside the company, however, could prove to be the tipping point. Hundreds of Facebook employees signed a letter to Zuckerberg calling for the policy to be changed.

But Facebook, having backpedaled over scandals including Russian disinformation and the Cambridge Analytica data leak, has held firm — at least so far. While Zuckerberg made the splashy appearances, including a speech at Georgetown and a hearing before the House Financial Services Committee, Instagram head Adam Mosseri has fought in the trenches, engaging with journalists and critics on Twitter and appearing on The Bill Simmons Podcast to discuss the topic. Sheryl Sandberg, Facebook’s chief operating officer, appeared on Bloomberg TV to back up the company’s stance.

And Facebook does have some external backers, many of whom argue that the company is right — that banning political ads will help incumbents and limit options for activist organizations.

Facebook has at times taken a stand that has drawn criticism and public pressure, only to relent. The company declined to ban far-right radio host Alex Jones before eventually removing him from the platform.

“I would anticipate that Facebook will have to change its policies — at least, overtly, to alleviate the tremendous pressure they will feel now from politicians and the public alike,” Ghosh said.

While the argument over political ads can seem narrow, tech industry activists have heralded Twitter’s move as an acknowledgement of their more systemic critiques about social media.

For Tristan Harris, a former Google employee who is the co-founder and executive director of the Center for Humane Technology, a nonprofit made up of former tech workers concerned about smartphone addiction and other social effects of technology, it goes well beyond Dorsey vs. Zuckerberg.

“This is the first step,” Harris said. “This is one of those moments where it really depends on everybody jumping into this.”