Former Vice President Joe Biden issued an open letter to Facebook’s Mark Zuckerberg on Thursday, calling for the social media giant to stamp out misinformation, including proposing a two-week pre-election period where all political ads are fact-checked.

Biden, the presumptive Democratic presidential nominee, also called for “clear rules — applied to everyone, including Donald Trump — that prohibit threatening behavior and lies about how to participate in the election.”

“Anything less will render Facebook a tool of misinformation that corrodes our democracy,” the letter reads.

Facebook responded to Biden, saying “we will protect political speech, even when we strongly disagree with it.”

“We live in a democracy, where the elected officials decide the rules around campaigns. Two weeks ago the President of the United States issued an executive order directing Federal agencies to prevent social media sites from engaging in activities like fact-checking political statements,” Facebook said in a statement.

“This week, the Democratic candidate for President started a petition calling on us to do the exact opposite. Just as they have done with broadcast networks — where the U.S. government prohibits rejecting politicians’ campaign ads — the people’s elected representatives should set the rules, and we will follow them.”

The open letter comes just weeks after the platform refused to take action on Trump’s posts about protests across the country, one of which was widely seen as threatening violence.

On May 29, Trump tweeted, “Any difficulty and we will assume control but, when the looting starts, the shooting starts,” a phrase used by a racist police chief in Florida in 1967.

Twitter appended a warning label to the tweet that said it “glorifies violence,” but Facebook took no action on an identical post that was put on the president’s official Facebook page.

Facebook’s decision not to take action sparked criticism from inside and outside the company. Some Facebook employees staged a virtual walkout days later, railing against the decision. One employee called for Zuckerberg to “immediately take down the President’s post advocating violence, murder and imminent threat against Black people.” Several other employees have quit in protest.

The post remains live on Facebook.

Days after Twitter took action on Trump’s tweet, the president signed an executive order asking federal agencies to look into companies for curating content. Legal experts largely do not believe the executive order is enforceable.

Biden’s letter said that Facebook “has taken no meaningful action” since the 2016 election, when the site was plagued by foreign and domestic disinformation campaigns.

“With fewer than five months until the 2020 election, real changes to Facebook’s policies for their platform and how they enforce them are necessary to protect against a repeat of the role that disinformation played in the 2016 election and that continues to threaten our democracy today,” a Biden campaign spokesperson said in a statement.

Trump and Zuckerberg have talked several times about the company’s role in content enforcement, including a phone call shortly after Trump’s post about shooting looters. At least one of the meetings, an October dinner with Trump, Zuckerberg and Facebook board member and Trump donor Peter Thiel, was not revealed until NBC News confirmed the meeting one month later.

Marianna Sotomayor contributed.