Twitter is banning all political advertising worldwide, saying: "We believe political message reach should be earned, not bought."
In a thread, CEO Jack Dorsey insisted the move was not about "free expression" but about protecting the public.
He wrote: "A political message earns reach when people decide to follow an account or retweet. Paying for reach removes that decision, forcing highly optimized and targeted political messages on people. We believe this decision should not be compromised by money.
"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.
"Internet political ads present entirely new challenges to civic discourse: machine learning-based optimization of messaging and micro-targeting, unchecked misleading information, and deep fakes. All at increasing velocity, sophistication, and overwhelming scale."
The company's decision comes against the backdrop of growing concern about the influence social media plays in elections, particularly following the 2016 US presidential elections, when Russia is believed to have used sites including Twitter and Facebook to try to influence the result in favour of Donald Trump.
Facebook has been criticised for saying this month that it will not fact-check ads by politicians or their campaigns, which could allow them to lie freely. CEO Mark Zuckerberg told Congress last week that politicians have the right to free speech on Facebook.
Last month sites including Twitter, Facebook and Google refused to remove a misleading video advert from Mr Trump's campaign that targeted former vice-president Joe Biden, one of the frontrunners in the race to win the Democratic nomination.
In response, Elizabeth Warren, another Democratic presidential hopeful, made a point by deliberately running a false advert on Facebook that claimed Mr Zuckerberg was endorsing President Trump for re-election.
Critics have called on Facebook to ban all political adverts.
CNN chief Jeff Zucker said the policy of allowing lies was ludicrous and advised Facebook to sit out the 2020 election until it can produce a more effective model.
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