Facebook told Joe Biden’s campaign this week that the Trump campaign’s ad pushing the debunked conspiracy theory about Biden and Ukraine will not be removed from its platform.

In a letter obtained by TPM (and first reported by the New York Times), Facebook Global Politics and Government Outreach Director Katie Harbath explained the social media giant’s decision to the Biden campaign.

“Our approach is grounded in Facebook’s fundamental belief in free expression, respect for the democratic process, and the belief that, in mature democracies with a free press, political speech is already arguably the most scrutinized speech there is,” Harbath wrote. “Thus, when a politician speaks or makes an ad, we do not send it to third party fact checkers.”

Harbath said the exception would be if a politician shares a “viral hoax” in the form of an article, video, or photo “that has previously been debunked.”

“That is different from a politician’s own claim or statement–even if the substance of that claim has been debunked elsewhere,” she wrote.

According to Harbath’s LinkedIn profile, the Facebook executive previously worked for the National Republican Senatorial Committee and Rudy Giuliani’s 2008 presidential campaign.

Biden campaign Press Secretary TJ Ducklo blasted Facebook’s decision in an emailed statement to TPM.

“It is unacceptable for any social media company to knowingly allow deliberately misleading material to corrupt its platform,” Ducklo said. “Donald Trump has demonstrated he will continue to subvert our democratic institutions for his own personal gain, but his shortcomings are no excuse for companies like Facebook to refuse to do the right thing.”

Facebook spokesperson Devon Kearns defended the decision, pointing out that other social platforms and media outlets “have made a similar decision to allow the ad to run.”

Last week, CNN said it would not air the ad on its channel because the ad both attacks CNN’s reporters and “makes assertions that have been proven demonstrably false.”

Read the letter below: