Twitter has banned all political advertising worldwide, saying that the reach of such messages "should be earned, not bought".
"While internet advertising is incredibly powerful and very effective for commercial advertisers, that power brings significant risks to politics," company CEO Jack Dorsey tweeted.
Social media rival Facebook recently ruled out a ban on political ads.
Social media firms are under particular scrutiny ahead of US elections in 2020.
Mr Dorsey explained his position in a thread of tweets.
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
How does Dorsey justify the ban?
Internet political ads, he said, presented "entirely new challenges to civic discourse".
These challenges included "machine learning-based optimisation of messaging", "micro-targeting, unchecked misleading information, and deep fakes".
"It's not credible," he wrote, "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!'"
Countering the argument that the new policy might be seen as favouring leaders already in office, he pointed out that "many social movements reach massive scale without any political advertising".
Ads in support of voter registration would not be affected by the ban, he added.
Hillary Clinton, the former Democratic candidate who lost to Donald Trump in the 2016 presidential election, welcomed Twitter's ban and appeared to challenge Facebook to rethink its stance.
Social media analyst Carl Miller said it was "one of the first times a tech giant has stepped back in concern for the enormous disruptions they're doing to the institutions that don't move as quickly as them".
What is Facebook's policy?
Facebook founder Mark Zuckerberg went before an audience of students in Washington DC earlier this month to defend the firm's decision not to ban political adverts that contain falsehoods.
He had considered barring all political ads on his platforms but said he believed the move would favour incumbent politicians and whoever the media chose to cover.
The company should "err on the side of greater expression", he argued.
A spokesman for Democratic presidential hopeful Joe Biden had criticised the firm for refusing to remove a video posted by Mr Trump's 2020 re-election campaign which promoted an unproven conspiracy theory involving the former vice-president and his son.
"It is unacceptable for any social media company to knowingly allow deliberately misleading material to corrupt its platform," Mr Biden's press secretary said.
Fellow Democratic candidate Senator Elizabeth Warren then paid to run an intentionally misleading advert on Facebook that claimed Mr Zuckerberg had personally endorsed Donald Trump for re-election.
She said she had done so in protest against the firm's decision to allow politicians to run ads containing" known lies".
How much impact will this have on the US election?
US federal campaigns are expected to spend about US$6bn on advertising but most of it will go on TV ads, with about 20 percent put into digital ads, advertising research firm Kantar estimates.