(CNN) Facebook denied a request from Joe Biden's campaign to take down a video ad by President Donald Trump's reelection campaign that falsely accuses the former vice president of corruption for his role in Ukraine policy during the Obama administration.

In the letter obtained by CNN, Facebook responds to the campaign's request by saying the decision is "grounded in Facebook's fundamental belief in free expression, respect for the democratic process, and belief that in mature democracies with a free press, political speech is already arguably the most scrutinized speech there is." Facebook adds that they do not send ads to third party fact-checkers. The New York Times first reported the letter from Facebook.

The video, which has been also posted by the campaign to Twitter and YouTube, accuses Biden of using his power as vice president to influence Ukraine in order to benefit his son, Hunter. There is no evidence of wrongdoing by Joe or Hunter Biden. CNN has refused to air the ad.

The Biden campaign responded to the Facebook letter by calling the decision "unacceptable."

Campaign spokesman TJ Ducklo said in a statement that the spread of "objectively false information to influence public opinion poisons the public discourse and chips away at our democracy."

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