"We believe political message reach should be earned, not bought," Dorsey said in a series of tweets. "While internet advertising is incredibly powerful and very effective for commercial advertisers, that power brings significant risks to politics, where it can be used to influence votes to affect the lives of millions."

We’ve made the decision to stop all political advertising on Twitter globally. We believe political message reach should be earned, not bought. Why? A few reasons…🧵 — jack 🌍🌏🌎 (@jack) October 30, 2019

Twitter is removing itself from the contentious tangle embroiling Facebook and its CEO Mark Zuckerberg as U.S. political campaigns prepare to spend vast sums of money on online advertising around the 2020 elections.

Spending on political advertisements overall could reach as much as $6 billion in the 2020 election cycle, according to projections from Advertising Analytics and Cross Screen Media, firms that analyze the advertising market. They predict $1.6 billion will be spent on digital video, with Facebook and Google collecting the bulk of that money.

So far, Facebook has raked in $857 million on political and issues-based advertisements since May 2018, the company has disclosed. Google and its video platform YouTube have pulled in $122 million from adds featuring politicians holding or seeking federal office during that same period. A Twitter spokesperson did not respond to a request for information on how much revenue political advertising generates for the social network. The company previously disclosed that it brought in roughly $3 million in political advertising revenue during the 2018 midterm election cycle.

Twitter's new policy applies worldwide, not just in the U.S., and to issue ads as well as ads run by specific political campaigns.

Dorsey didn't name Zuckerberg in his series of tweets announcing the change, which takes effect Nov. 22. But he didn't shy away from poking holes in the defense Zuckerberg has offered of Facebook's controversial handling of political ads.

Dorsey, for example, tweeted, "This isn’t about free expression. This is about paying for reach." Zuckerberg gave a high-profile speech at Georgetown University two weeks back in which he cited his company's commitment to "free expression" as the reason for allowing misleading political ads on its site.

The Twitter chief also took aim at the Zuckerberg-promoted claim that tightening policies around political ads would disproportionately hurt challengers and grassroots groups. Dorsey said that "we have witnessed many social movements reach massive scale without any political advertising."

And he mocked the prospect of continuing to adopt Facebook-style positions on misinformation. "[I]t‘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.

Clinton made text of the subtext, directly pressing Facebook to answer Twitter's challenge. "This is the right thing to do for democracy in America and all over the world. What say you, @facebook?" tweeted Clinton, the target of a vast Russia-linked misinformation that flourished on Facebook and Twitter in the 2016 election.

Zuckerberg, though, dug in on Facebook's latest quarterly earnings call, which happened to start just an hour after Dorsey announced the new policy. He didn't foreclose the possibility of ever reconsidering, saying he'll continue to assess "whether we should not carry these ads" but has decided "on balance" that it's the right call.

"Right now, the content debate is about political ads. Should we block political ads with false statements? Should we block all political ads?" Zuckerberg said. "In a democracy, I don't think it's right for private companies to censor politicians or the news."

Facebook has come under intense scrutiny in recent weeks for allowing Trump’s reelection campaign and its supporters to run advertisements on the platform that assert, without evidence, a corrupt relationship between former Vice President Joseph Biden and the government of Ukraine.

Biden’s presidential campaign has condemned Facebook for permitting deliberately misleading advertisements, including one that CNN had rejected due to its inaccuracy. Other Democratic candidates, including Massachusetts Sen. Elizabeth Warren, have also strongly criticized Facebook’s approach — one the company has defended by arguing that it should not be a gatekeeper for political speech online.

The Biden campaign offered wary partial approval to Twitter's new policy Wednesday. "It would be unfortunate to suggest that the only option available to social media companies to do so is the full withdrawal of political advertising," said deputy communications director Bill Russo, "but when faced with a choice between ad dollars and the integrity of our democracy, it is encouraging that, for once, revenue did not win out."

Progressive New York Rep. Alexandria Ocasio-Cortez was less ambivalent,tweeting, "Not allowing for paid disinformation is one of the most basic, ethical decisions a company can make."

While much of the ire over false political advertising has been focused on Facebook, Twitter has not escaped reproach for its handling of political speech. Democrats have argued that Twitter should take down tweets from President Trump that seem to encourage the harassment of his political enemies or threaten entire countries. California Democratic senator and presidential candidate Kamala Harris has called on Twitter to kick Trump off the service altogether.

Twitter has argued that world leaders' tweets are worth keeping up because they're newsworthy, even if they violate the site's terms of service.

Cristiano Lima contributed to this report.