The 30-second video ad released by the Trump campaign last week is grainy, and the narrator’s voice is foreboding. Former vice president Joe Biden, it says, offered Ukraine $1bn (£820m) in aid if the country pushed out the man investigating a company tied to Mr Biden’s son.

Saying it made false accusations, CNN immediately refused to air the advertisement.

But Facebook did not, and Tuesday, the social network rejected a request from Mr Biden’s presidential campaign to take it down, foreshadowing a continuing fight over misinformation on the service during the 2020 election as well as the impeachment inquiry into president Donald Trump.

In a letter to the Biden campaign, Facebook said the ad, which has been viewed 5 million times on the site, did not violate company policies. Last month, the social network, which has more than 2 billion users, announced that politicians and their campaigns had nearly free rein over content they post there.

Even false statements and misleading content in ads, the company has said, are an important part of the political conversation.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

“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 scrutinised speech there is,” Facebook’s head of global elections policy, Katie Harbath, wrote in the letter to the Biden campaign.

The decision by the company illustrates its executives’ hardened resolve to stay out of the moderation of political speech, despite the use of the social network to spread discord and disinformation in the 2016 presidential campaign. On Tuesday, the Republican-led Senate Intelligence Committee released a sobering report warning of fresh signs of interference by Russia and other foreign nations in the 2020 election.

The company’s position stands in contrast to CNN, which rejected two ads from the Trump campaign last week, including the one the Biden campaign asked Facebook to take down. The cable channel said it rejected the ad because it “makes assertions that have been proven demonstrably false by various news outlets.”

Facebook has been dogged by accusations of censorship by conservative politicians, including Mr Trump, who argue that the Silicon Valley company gives greater attention to liberal points of views on the social network.

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 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 27 September, starts with staticky shots of Biden meeting with Ukrainian officials during his time in the Obama administration.

“Joe Biden promised Ukraine $1bn if they fired the prosecutor investigating his son’s company,” a narrator says, using video from an event in which 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.

Schultz wrote that the vice president’s call for a new prosecutor in Ukraine’s investigation of a company was supported by the European Union, the International Monetary Fund and the World Bank. He included clippings from The Washington Post and Factcheck.org that debunked the claims of Mr Biden’s motives of squashing the investigation to benefit his son.

After CNN rejected the ad last week, Tim Murtaugh, communications director for Trump’s campaign, said that the ad was “entirely accurate and was reviewed by counsel.”

“CNN spends all day protecting Joe Biden in their programming,” Mr Murtaugh wrote in a statement. “So it’s not surprising that they’re shielding him from truthful advertising, too.”

The Biden campaign also urged Fox News to reject the Trump campaign ad last week. But the cable channel declined to do so, saying that it was “not in the business of censoring ads from candidates on either sides of the aisle.”

The ad has also appeared on YouTube and Twitter. A spokesman for Twitter said Tuesday that the ad complied with its policies. A YouTube official likewise said the ad complied with its policies.

TJ Ducklo, a spokesman for Mr Biden, wrote in an email that “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.”