But by removing itself as the moderator of political content — including in paid ads on the site — Facebook has left itself open another avenue of criticism. In a series of tweets Monday evening, Senator Elizabeth Warren, one of the front-runners for the Democratic nomination, said Facebook allowed President Trump to spread false information widely, and called on the company to take down the attack ad against Mr. Biden, one of her top rivals.
“Facebook already helped elect Donald Trump once because they were asleep at the wheel while Russia attacked our democracy — allowing fake, foreign accounts to run ad campaigns to influence our elections,” Ms. Warren wrote.
Facebook declined to comment.
The ad the Biden campaign asked Facebook to take down, released by the Trump campaign on Sept. 27, starts with staticky shots of Mr. Biden meeting with Ukrainian officials during his time in the Obama administration.
“Joe Biden promised Ukraine $1 billion if they fired the prosecutor investigating his son’s company,” a narrator says, using video from an event in which Mr. Biden mentions the money. “But when President Trump asks Ukraine to investigate corruption, the Democrats want to impeach him.”
The $1 billion figure was mentioned at an event in 2018 at the Council on Foreign Relations, in which Mr. Biden was talking about how the Obama administration tried to root out corruption in Ukraine. He said he had held back financial aid to push the country to make reforms.
There is no evidence that Mr. Biden, during his time as vice president, pushed for the dismissal of the Ukrainian prosecutor general to help his son Hunter Biden. The former vice president, along with other members of the Obama administration and other international leaders, pushed for the removal of the prosecutor, Viktor Shokin, because of accusations that he ignored corruption.
Days after the ad was broadcast on television and social networks the Biden campaign wrote a letter to Mark Zuckerberg, Facebook’s chief executive, and Sheryl Sandberg, the chief operating officer, asking to reject the ad. The video “spreads false, definitively debunked conspiracy theories regarding Vice President Joe Biden,” Greg Schultz, a Biden campaign manager, wrote to the Facebook leaders.