Among those potentially affected could be public-interest nonprofits eager to reach an audience larger than their official followers, challengers to incumbent officeholders, and — obviously — political consultants who make a living placing ad buys for their candidates.
Twitter CEO Jack Dorsey said in a series of tweets that paid political messages in the targeted environment that social media enables can be fraught. "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," he wrote.
Security and privacy researchers and some Democratic politicians hailed Twitter's decision as an important way to prevent campaigns from feeding streams of misinformation to targeted voters. The move drew a sharp contrast between Twitter and its much larger rival Facebook, which has come under fire in recent months for its policy of not fact-checking political ads.
Facebook CEO Mark Zuckerberg shot back quickly, using an earnings conference call Wednesday afternoon to offer an impassioned defense of what he called his company's deep belief "that political speech is important."
"This is complex stuff. Anyone who says the answer is simple hasn't thought about the nuances and downstream challenges," Zuckerberg said. "I don't think anyone can say that we are not doing what we believe or we haven't thought hard about these issues."
Google did not have an immediate comment on Twitter's policy change. Trump's campaign manager called Twitter's change a "very dumb decision" in a statement Wednesday. "This is yet another attempt to silence conservatives, since Twitter knows President Trump has the most sophisticated online program ever," campaign manager Brad Parscale said.
Political advertising makes up a small sliver of Twitter's overall revenue. The company does not break out specific figures each quarter, but said political ad spending for the 2018 midterm election was less than $3 million. It reported $824 million in third-quarter revenue.
Candidates spend significantly more purchasing ads on Facebook than on Twitter, company records show. In a semi-annual report on enforcing its guidelines, Twitter said Thursday that more than 50% of the tweets it removes for abuse are now "proactively" flagged using technology such as artificial intelligence rather than needing someone to report problems. That compares with 20% a year ago, it said.
"Our continued investment in proprietary technology is steadily reducing the burden on people to report to us," Twitter said. The report, covering the first six months of the year, also said accounts either locked or suspended for violating Twitter's rules more than doubled from the previous six months.
The political advertising issue rose to the forefront earlier this fall when Twitter, along with Facebook and Google, refused to remove a misleading video ad from President Donald Trump's campaign that targeted Democratic presidential candidate Joe Biden.
In response, Democratic Sen. Elizabeth Warren, another presidential hopeful, ran her own ad on Facebook taking aim at Zuckerberg. The ad falsely claimed that Zuckerberg endorsed President Donald Trump for re-election, acknowledging the deliberate falsehood as necessary to make a point.
Dorsey said the company is recognizing that advertising on social media offers an unfair level of targeting compared to other mediums. It is not about free expression, he asserted. "This is about paying for reach. And paying to increase the reach of political speech has significant ramifications that today's democratic infrastructure may not be prepared to handle," he tweeted. "It's worth stepping back in order to address."
Zuckerberg said he has also considered banning political ads, but remains wary of the move's impact. "It's hard to define where to draw the line," he said. "Would we really block ads for important political issues like climate change or women's empowerment?"
A ban on such "issue ads" could limit the ability of such groups to reach wider audiences or disadvantage them in other ways. Ryan Schleeter, a spokesman for the environmental group Greenpeace, said a lot will depend on how Twitter defines "political."
What the group doesn't want to see, Schleeter said, is major oil companies being able to run misleading ads, while those who confront them are censored. Political challengers will also find likely find themselves at a disadvantage, since they don't generally have the name recognition or money that their opponents do, said Matt Shupe, a Republican political strategist.
"If you're a challenger, advertising allows you to make up that difference," he said. "It's very hard to organically grow an audience for a state assemblyman campaign." Shupe, whose public relations firm has won awards for its use of ads on Facebook, called Twitter's decision "incredibly dumb."
Twitter said it will make some exceptions, such as allowing ads that encourage voter registration. It will describe those in a detailed policy it plans to release on Nov. 15, and the policy will take effect Nov. 22.
Twitter will still allow politicians to freely tweet their thoughts and opinions, which can then be shared and spread. Trump's Twitter feed in particular is known for often bombastic and controversial tweets that are shared widely.
Federal campaigns are expected to spend the majority of their advertising dollars on broadcast and cable channels during the 2020 election, according to advertising research firm Kantar. About 20% of their total $6 billion in ad spending are expected to be on digital ads.
AP reporters David Klepper in Providence, Rhode Island, Amanda Seitz in Chicago, Will Weissert in Washington, Mae Anderson in Atlanta and Tali Arbel in New York, and Marcy Gordon in Washington contributed to this article.
This story has been corrected to state that campaigns are expected to spend most of their ad dollars on broadcast and cable channels, rather than accounting for the majority of dollars on those channels.