The 2016 presidential campaign showcased the worst of Facebook. Remember the headline “Pope Francis shocks world, endorses Donald Trump for president”? That story had nearly 1 million Facebook impressions by Election Day, prompting the pope to call such fake news a “sin” and a “sickness.”

That wasn’t the worst. There was much to dislike, starting with massive evidence that Russian operatives pumped out swaths of false or unflattering posts about presidential candidate Hillary Clinton, to promote Trump and to foment division over race, the criminal justice system, immigration and more. The Russian campaign was an assault on American democracy that Facebook should have prevented.

Facebook did make some changes when the extent of the Russian propaganda effort became clear after the election. In early 2018, it tweaked its newsfeed algorithms to promote content that co-founder Mark Zuckerberg said was more likely to be “good for people’s well-being.” It also began requiring that all political ads be labeled with a “paid for by” disclosure tag. Earlier this year, Facebook introduced even stricter rules on verifying ad sponsors.

A new Facebook decision, though, could play havoc with the 2020 election. Facebook executive Nick Clegg announced that the company’s 30,000 contracted content moderators are no longer fact-checking political ads, which he said amounted to censorship. Moderators will block “previously debunked content” — but fresh lies will be allowed.

As Harvard law Prof. Cass Sunstein noted, having the company stay out of fact-checking will make it less likely to face claims of political bias for keeping certain allegations off its platform. A recent independent audit by former Sen. Jon Kyl, R-Ariz., that Facebook had commissioned wasn’t damning, but it did show a content review process that appeared arbitrary — inviting suspicion and fear of bad faith. Zuckerberg may have concluded this is one headache Facebook can do without.

But is that his actual motive? Or does the Facebook CEO relish the idea that Trump will viciously go after Democratic presidential candidate Elizabeth Warren and others who want to break up Facebook? In a tape of a meeting with employees that was leaked to the Verge, Zuckerberg said, “If someone’s going to try to threaten something that existential, you go to the mat and you fight.”

Facebook is already running a Trump campaign ad that was rejected by CNN and NBC. It flatly asserts then-Vice President Joe Biden pressured Ukraine to fire a prosecutor so as to end an investigation of his son, Hunter Biden. But there was strong evidence that Washington wanted the prosecutor fired because he was ignoring corruption — not because of Hunter Biden.

Facebook can’t fact-check every post, but it should resume fact-checking political ads. The heat it might face if it adopted a higher standard won’t be nearly as severe as the backlash it would invite if Facebook is seen as Donald Trump’s secret weapon — and silent partner — in the 2020 campaign.