"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.
"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!'"